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The Big Bang | Rogue Planets | Galaxy Collisions | The End Of The Universe

WELCOME

Space is an area of exploration that captures the imaginations of so many people. Throughout history, valiant astronauts, engineers, astronomers and scientists have made countless landmark revelations that help us understand our place in the universe. With all that said, however, there are so many unanswered questions and discoveries yet to be revealed that have us on the edge of our seats.
In this bookazine, we will put many of those mysteries under the microscope. Inside, you will explore the mysteries surrounding the Big Bang, examine galactic enigmas like black holes and quasars and interrogate conundrums closer to home, such as the formation of our Moon. That’s not all, though: we’ll delve into the ever-tantalising search for alien life and ponder whether humanity will set foot on another planet before we get existential with the end of the universe and the theory of whether we are living in a multiverse.
So, prepare for blast-off and get ready to decode some of the biggest mysteries in the universe!

© Adobe Stock; Shutterstock

28

Contents

Cosmic foundations
Delve into the secrets behind the universe’s origins and its elemental forces
08...Mysteries of the big bang Welcome to the enigmatic and all-important birth of the universe
16...Dark forces The difficult mission to uncover the true nature of dark matter and dark energy
22...Mysteries of time Expand your perception of time beyond the human experience
The solar system Unravel the mysteries surrounding our local stellar neighbourhood
28...How was the moon formed? Discover what might be the most important collision in Earth’s history
34...Planet nine Is there a covert planet exerting its influence beyond Neptune?
40...mysteries of mars Unpacking the secrets of the Red Planet could set us up for living on it
46...can saturn’s moons sustain life? Enceladus and Titan are prime candidates for microscopic life
50...the oort cloud At the fringes of our Sun’s influence lies this secretive time capsule
54...will humans set foot on  another planet? Recent advancements in space flight give us cause to be excited
Galactic enigmas
Explore beyond our solar system and into the wider universe
62...How are galaxies formed? Astronomers can now paint a picture of the origins of star systems
68...Black holes Prepare to have your mind blown by these light-devouring phenomena
74...Quasars The cosmic lighthouses of the universe have their share of secrets
78...the great attractor theory Galaxies are being drawn to a specific region of space… but by what?
82...When Galaxies collide These cosmic crashes may provide clues to how today’s universe formed
Alien Worlds
Is humanity alone or are there extraterrestrials on extrasolar planets?
88...Exoplanets These strange new worlds hold limitless possibilities
94...rogue planets What secrets do these lone wanderers hold?
98...the search for alien life Explore the possibilities of humans ever encountering life beyond Earth
104...The fermi paradox If there are aliens out there… why haven’t they contacted us?
at the Edge
Decode some of the biggest questions that have experts scratching their heads
110...How large is the universe? Crunch the numbers on our ever-expanding universe
114...How will the universe end? All good things must end… and it’s best to be prepared
120...Welcome to the multiverse You might not be the only you reading these words right now
124...Space’s biggest mysteries (that we’ve solved) Astronomy has come a long way, so let’s relive some milestones

34

22

68

88

120

74

94

98

© Adobe Stock; Getty; Alamy; NASA

Delve into the secrets behind the universe’s
origins and its elemental forces

16

8

8 Mysteries of the big bang
16 dark forces
22 Mysteries of time

© Adobe Stock; Getty; Alamy

Humanity has encountered many mysteries
surrounding the universe, but none is
more important than the Big Bang. Read
on to discover more about the universe’s
bombastic beginnings

Written by Katherine Bebo

Google ‘the Big Bang theory’, and we imagine the first couple of results will be all about the American sitcom that follows the lives of a group of science geeks that started airing in the 2000s. Well, the actual big bang is much more impressive – and considerably older. After all, 13.8 billion years – when this massive explosion is alleged to have happened – is a pretty long time ago. And without it, the world as we know it would not exist. Indeed, without the Big Bang, there would be no stars, no galaxies, no planets, no Earth, no humans… no nothing. There are many theories as to how the Big Bang occurred. Was it a collision? An energy spark? Was it an event within a multiverse? Perhaps a higher power instigated it? Here, we will explore all these questions – and more – plus we’ll gaze back at what potentially happened before and after the Big Bang took place.
We will look at the rapid expansion of the universe and question the fact that it’s actually still expanding. What if it never stops? Or, worse, what if it does? Is the Big Bang still happening? Will it ever reverse? Just like outer space is infinite, so are the number of questions it poses. It is home to the most intriguing, thrilling, bonkers stuff that ever existed, and delving into the world of outer space will open your eyes, boggle your brain and blow your mind. Ready to discover how it all started?

The name ‘Big Bang’ was
coined by Fred Hoyle
originally as a way to
ridicule the theory.

Mysteries of the big bang

© Adobe Stock; Getty

What Caused

The big bang?

Imagine nothing: a complete absence of matter. This is what our universe was – or wasn’t – billions and billions of years ago. Or was it? The question of what existed before the Big Bang has had scientists scratching their heads since the theory’s inception, and it continues to be a hotly debated scientific topic today. Indeed, perhaps there was nothing, or maybe another universe existed – or a different version of our own. Maybe a plethora of universes existed, each with a different set of physical laws. The fact is that no one can definitively say for sure. What we do know, however, is that our universe as we know it didn’t exist before the Big Bang… and now it does. So the question remains: how did our astounding, mind-boggling, terrifying, glorious, overwhelming, wondrous, dramatic, tremendous, immense universe come to exist? Many fascinating theories of what caused the Big Bang are out there.
The first we will look at is the concept of quantum fluctuations – random changes in the energy levels of points in space. The idea here is that the Big Bang emerged from a quantum fluctuation in a preexisting ‘quantum vacuum’. In quantum physics, even a vacuum is not truly empty but contains temporary particles and energies popping in and out of existence. As such, a tiny fluctuation – or random energy spark – might have created the Big Bang.
Meanwhile, the Big Bounce theory suggests that the Big Bang wasn’t the beginning of the universe but rather a transition. Instead of the universe starting from nothing, the Big Bounce proposes that before the Big Bang, the universe was in a previous phase where it had been shrinking or collapsing. As it collapsed into an incredibly dense, tiny state, instead of being the end of everything, the universe ‘bounced’ back and started expanding outwards. This rebound – the ‘bounce’ – was the Big Bang. This idea suggests that the universe may go through endless cycles of collapse and rebirth. The number of cycles the universe has lived through, really, is anyone’s guess.
The Brane Collision theory suggests that the Big Bang was caused by a collision between two massive, invisible ‘branes’ (short for membranes) floating in higher-dimensional space. These branes were like giant, flexible sheets moving through extra dimensions. When two branes get too close and collide, the energy from their collision creates a significant explosion –  the Big Bang.
But what if our universe isn’t the only one? Some scientists think there’s a ‘multiverse’, like an infinite factory constantly creating new universes. In this idea, the Big Bang was just one of these creations, a smaller universe born out of this bigger system. Maybe in another universe, gravity works differently, there’s no light, or timelines work discordantly. The multiverse theory implies that the Big Bang is just one of many similar events, with universes constantly forming, evolving and existing alongside each other.
Theologians believe that the Big Bang was the result of an intentional act of creation by a divine or supernatural being, often referred to as God. Following this view, the Big Bang marks the moment when the universe – including space, time and, well, everything – was brought into existence by this being. The theory focuses more on answering the ‘why?’ behind the Big Bang rather than the ‘how?’. The laws of physics, the fine-tuning of the universe, and the emergence of life are often seen as evidence of a purposeful design: things work so seamlessly together – this theory posits that it can’t be dumb luck.
These theories are all very well and good, but author Terry Pratchett perhaps summarised things aptly in his novel Lords And Ladies: “The current state of knowledge can be summarised thus: in the beginning, there was nothing, which exploded.”

Theorists argue that
our universe is one
of many in a cycle of
death and rebirth.

A diagram showing
the evolution of the
universe after
the Big Bang.

Gravity pulled gas
together kindling the
first stars, which then
formed galaxies.

A brief

History
of the
universe

“our universe
as we know it
didn’t exist
before the
Big Bang”

The Big Bang
(13.8 billion years ago) The universe begins as a hot, dense, tiny point. In a fraction of a second, cosmic inflation occurs, expanding exponentially and setting the stage for everything that follows.
Formation of Atoms
(380,000 years after the Big Bang)
As the universe cools, protons and electrons combine to form the first atoms, mostly hydrogen and helium.
Birth of Stars and Galaxies
(200 million years after the Big Bang)
Gravity pulls gas clouds together, igniting the first stars. Groups of stars form the earliest galaxies, lighting up the once-dark universe.
Start of the Solar System
(4.6 billion years ago)
Our sun and planets form from a swirling cloud of gas and dust within the Milky Way galaxy.
Rise of Life on Earth
(3.5 billion years ago)
Simple life forms appear on Earth, evolving over billions of years into the complex ecosystems we see today. Humans pop up roughly 300,000 years ago.
Universe Now and in the Future
The universe continues to expand. Its fate depends on dark energy, which could either cause endless expansion or a doomy end like the Big Crunch.

© Adobe Stock; Getty; NASA

Cosmic Inflation

The Universe’s Lightning-Fast Growth Spurt

According to the Big Bang theory, the universe began as a singular point. Everything was packed extremely close together, and after the explosion, all the matter began racing outwards, speeding out in all directions. In the first moments after the Big Bang, the universe was about 5.5 billion Celsius (10 billion Fahrenheit) and ballooned faster than the speed of light. During this time, the universe doubled in size at least 90 times. In less than the blink of an eye, a speck smaller than the size of a proton expanded to the size of a grapefruit. This phenomenon has been named cosmic inflation.
The rapid stretching of the universe smoothed out any irregularities, making it uniform in all directions. After the period of rapid cosmic inflation ended, the universe continued expanding at a slower pace, cooling down enough for matter and radiation to form the beginnings of galaxies. Much like many things in our profoundly perplexing cosmos, the exact cause of inflation is still debated among scientists, but it likely resulted from a special kind of energy field called the ‘inflaton field’, which dominated the universe in its earliest moments. The energy in this field caused space itself to stretch exponentially. When inflation ceased, the energy in the field converted into particles, creating the hot soup of matter and radiation that followed.
So, why is understanding cosmic inflation so important? Well, unlocking the key behind this phenomenon will help to ‘solve’ several major puzzles in cosmology. It would explain ‘the horizon problem’. As we previously mentioned, the universe is remarkably uniform in all directions, but without inflation, distant regions of the universe couldn’t have interacted to even out their temperatures. The inflation theory, however, explains how they could have been closer.
Inflation explains why the universe appears flat (instead of curved) by stretching space to a scale so large that any curvature becomes negligible.
It also explains why we don’t see certain heavy particles predicted by early-universe theories, as inflation would have diluted them to near nonexistence. The universe began as a single point of tightly packed ‘stuff’, then began expanding and didn’t stop. So, how did stars, planets, comets, asteroids, galaxies and everything else in space come to be? When the universe began its journey, it was just a load of tiny, hot particles (neutrons, electrons and protons) mixed with energy and light. As things started to expand and take up more space, they reduced in heat. The particles grouped together, forming atoms. Then, those atoms grouped together. Eventually, groups of atoms formed stars and galaxies. These first stars created larger, more complex atoms, then groups of atoms called molecules. Cue more stars being made. Concurrently, galaxies were grouped together. As new stars were being born and dying, cosmic cool stuff like planets, asteroids, comets and black holes formed.
It’s all utterly confounding, and no one can fully explain how everything came into existence or how the universe multiplied the way it did. But, what can broadly be agreed upon is that cosmic inflation was a brief but critical phase that laid the groundwork for the gargantuan universe we know today.
While the details are still being refined, debated and reviewed, and many questions remain unanswered, inflation is one of the best explanations for the universe’s rapid and astonishing beginnings.

“Inflation
explains why the
universe appears
flat instead
of curved”

universe flat?

Is the

In a word, yes. On second thoughts, probably. The Wilkinson Microwave Anisotropy Probe (WMAP) has provided data to confirm that the universe is flat, with a margin of error of 0.4%. If the universe does have a curvature, it’s too miniscule to detect using our current measuring implements. While the universe is unimaginably big, its shape is pretty straightforward: flat, infinite and stretching out in all directions. It follows the basic rules of geometry: parallel lines stay parallel, and the angles of a triangle add up to 180 degrees.
Scientists figured this out by studying light left over from the Big Bang, called the cosmic microwave background (CMB). The patterns in this light – observed from satellites like the WMAP and Planck – show that the universe isn’t curved like a sphere or a saddle. It’s flat. It also means the universe has just the right amount of matter and energy to balance its expansion perfectly.

Mysteries of the Big Bang

It’s thought the universe
expanded exponentially
before slowing to
what we know today.

© Adobe Stock; Alamy; NASA

The universe is still
expanding to this
day, lending evidence
towards a Big Bang.

Universe

Breakers

There was once a time when everyone believed the Earth was flat. However, with the emergence of new information, this notion was scientifically debunked. Much more recently, new data has been captured that calls into question scientists’ understanding of how the earliest galaxies were formed. The James Webb Space Telescope (JWST) has detected so-called ‘universe breakers’ – six galaxies that seem impossibly massive or developed for their place in the timeline of the universe (they have been dated to a time when the universe was just 3% of its current age). These ‘universe breakers’ defy the expectations of how quickly stars, galaxies, and cosmic structures should have formed in the early universe, meaning our understanding of the first billion years of cosmic history and the universe’s evolution needs another look.

big bang

is the

Still happening?

One question that follows scientists today is whether the Big Bang is an ongoing phenomenon. Although the ‘main event’ of the Big Bang took place 13.8 billion years ago, its effects are still all around us today, and we’re enjoying the after party, refusing to collect our coats and hail a taxi. One way we’re still seeing the effects of the Big Bang is cosmic microwave background (CMB), a ‘leftover heat’ from the explosion that is still everywhere in the universe. Not only does this radiation help provide evidence that the Big Bang actually happened, but it also helps astronomers ascertain the age and composition of the universe.
Another after-effect of the Big Bang is that the universe is still expanding, stretching further out. The notion that the universe is expanding was developed in 1927 by Belgian astronomer (and priest) Georges Lemaître. Two years later, American astronomer Edwin Hubble noticed that other galaxies were moving away from us: the ones furthest away moving at a greater speed. This led to the conclusion that the universe is still expanding. This backed up the concept of the Big Bang: if things are currently moving apart, they must have, at one time, been very close together.
In the mid-1990s, it was found that the universe is expanding at a faster rate than originally thought, and some believe the reason for this is dark energy. This mysterious force that’s pushing everything apart could bring about the end of the universe. Scientists have been busy cooking up theories as to exactly how this could happen and trying to ascertain if the Big Bang could ever ‘reverse’.
The theory of the ‘Big Crunch’ explores that at some point, the universe will cease expanding, start shrinking and grow extremely hot and dense. Eventually, everything will be squished into a tiny point – much like how the Big Bang began – and collapse in on itself.
Another theory of the Big Freeze looks at if expansion continues, the universe could potentially keep stretching forever. Heat will be dispersed while stars, galaxies and planets are pulled apart. The heat will continue to decrease, and stars will run out of fuel until the temperature reaches absolute zero and nothing new can be born. Everything still existing will slowly be snuffed out, and everywhere will become dark, cold and lifeless: a state known as heat death.
Meanwhile, the Big Rip theory examines dark energy and whether it will cause expansion to speed up so much that it tears everything – galaxies, stars, planets, atoms – apart. Eventually, the universe reaches a point where all matter and energy are completely shredded, leaving nothing behind.
The last theory we’ll touch on here is the Big Slurp. This question is that we live in a fragile false vacuum that could be very unstable and might experience a catastrophic chain reaction where a an alternate true-vacuum universe interacts with our unstable one. If this other universe is of a lower energy state than ours then the energy from our higher energy universe will flow into the low-energy stable one. Interestingly, this theory posits that this could affect only part of our universe, compared to other theories which depict the doom of everything we know.
Scared yet? Try not to get too glum. Just like lives on Earth are unpredictable, the fate of the universe is no different. No one knows what the future holds for the universe billions of years down the line, so let’s try to enjoy the ride while we can.

“the universe
is expanding at
a faster rate
than originally
thought”

JADES-GS-z13-0 is a
high-redshift galaxy observed
by the JWST that existed
shortly after the Big Bang.

© Adobe Stock; Getty; NASA; ESA; CSA

Dark matter and dark energy make up 95%
of the cosmos, yet remain almost completely
shrouded in mystery. Understanding them could
be key to unlocking the secrets of ouruniverse.
So let’s try…

Written by Steve Charnock

Dark Forces

Imagine trying to solve an especially tough jigsaw puzzle when you only have 5% of the pieces in the box. Kind of tricky, right? Right. Well, that’s essentially what astronomers are up against when it comes to understanding the universe. Everything we can see, touch and measure - everything from stars, planets, meteorites, comets and galaxies - makes up just a tiny little fraction of what’s actually out there. The rest? It’s mostly dark matter and dark energy: two invisible forces shaping the cosmos in ways we are only now just beginning to comprehend.
Dark matter acts like an unseen adhesive, holding galaxies together and ensuring they rotate without flying apart. Without it, the universe could literally collapse in on itself. Quite the thought.
Dark energy, on the other hand, is even stranger. It’s the unseen controlling force behind the universe’s ever-accelerating expansion, seemingly defying gravity. Together, these dark and unknowable forces make up a staggering 95% of the universe’s total mass and energy.
Scientists have long been baffled by these phenomena. While their existence is inferred from their effects on the visible universe, they have never been directly detected. Making their study almost ludicrously complex.
Why is understanding dark matter and dark energy so critical? Because they could hold the answers to fundamental questions about the universe’s origin, evolution and even ultimate fate.
Recent breakthroughs are providing tantalising clues. With the aid of tech like the advanced James Webb Space Telescope and all the cutting-edge experimental work at CERN’s Large Hadron Collider, researchers are exploring new ways to illuminate these cosmic enigmas. The stakes couldn’t be higher, either: solving the mysteries of dark matter and dark energy could transform our understanding of physics, space, time and existence. So there’s no pressure, folks...

Dark forces research

Dark matter is,
fundamentally,
the invisible glue of the
universe. It’s also a whole
lot more than that, though.
Let’s investigate it...

Dark matter, one of the most elusive substances in the universe, remains an incredibly profound mystery in modern astrophysics. Though it neither emits or absorbs light – and cannot be observed directly – its existence is deduced through its gravitational effects on visible matter.
Accounting for approximately 85% of the total matter in the universe (and around 27% of the universe’s total energy density), dark matter is elemental to understanding the structure and evolution of the cosmos. Yet, despite decades of research into it, the true nature of dark matter continues to evade scientists. Unravelling the secrets of dark matter could lead to groundbreaking revelations about the universe’s origins, structure and ultimate fate.
The concept of dark matter was first proposed in the 1930s by Swiss astronomer Fritz Zwicky. While studying the Coma galaxy cluster, Zwicky noticed a large discrepancy: the visible mass of the galaxies was insufficient to generate the gravitational pull needed to hold the cluster together. The galaxies were moving so rapidly that, without an additional invisible mass, they should have dispersed into space. Zwicky dubbed this mysterious, unseen mass ‘dunkle materie’ or – as it’s known in the English language – ‘dark matter’.
Stab in the dark
Today, dark matter is considered a cornerstone of the standard model of cosmology, known as the ‘lambda cold dark matter’ mathematical model. It explains not only the motions of galaxies but also the large-scale structure of the universe, including galaxy clusters and cosmic filaments. Without dark matter, the universe as we know it simply wouldn’t have been able to form.
So what exactly is dark matter? Its exact nature remains one of the greatest unanswered questions in physics. Unlike ordinary matter, which consists of protons, neutrons and electrons, dark matter doesn’t interact with electromagnetic forces. This means it doesn’t emit, absorb or reflect light, making it invisible to traditional telescopes. Instead, dark matter interacts primarily through gravity (and possibly through weak nuclear forces). You can see why scientists have called ‘dark’, can’t you?
One leading theory is that dark matter is composed of exotic particles, such as ‘weakly interacting massive particles’ (WIMPs) or axions. WIMPs, as the name suggests, are particles that interact only weakly with ordinary matter, making them extraordinarily difficult to detect. Axions, on the other hand, are hypothetical particles with extremely low mass that may interact with electromagnetic fields, but only in subtle ways. Despite extensive research, neither WIMPs nor axions have been directly observed, leaving scientists to explore other possibilities, including alternative particles or entirely new physics.
In the spotlight
Dark matter is not just an isolated cosmic mystery – it’s key to answering some of the most fundamental questions about the universe. Its gravitational influence is essential to the formation of galaxies and galaxy clusters, meaning that without dark matter, the universe’s structure would look completely different. Studying dark matter could also reveal whether our current understanding of physics, particularly gravity, needs revision. Some alternative theories, such as modified Newtonian dynamics (MOND), attempt to explain dark matter’s effects without requiring ideas such as exotic particles, though these theories remain somewhat controversial in the astronomical community.
Solving the mystery of dark matter could open the door to new discoveries in particle physics, cosmology and beyond. If dark matter particles are discovered, they might reveal new forces of nature or connections to other phenomena, such as dark energy, which drives the accelerated expansion of the universe. As such, understanding dark matter is crucial not only for decoding the universe’s past but also for predicting its future.
While dark matter remains mysterious, the recent progress provides hope that its secrets may one day be unveiled. With advanced observatories, state-of-the-art experiments and innovative theoretical work, scientists are closer than ever to uncovering this hidden cornerstone of the cosmos. Solving the puzzle of dark matter could fundamentally reshape our understanding of the universe, revealing a deeper, more intricate reality than we ever imagined.

Dark matter is an
invisible force
that is key to our
universe’s structure.

Scientists believe dark
matter accounts
for 85% of total mass
in the universe.

“Its nature
remains one of
the greatest
unanswered
questions”

Gravitational Lensing

Seeing the Unseen

Gravitational lensing offers one of the most compelling ways for astronomers to study dark matter. When light from a distant galaxy passes near a massive object, such as a cluster of galaxies, the object’s gravitational field bends the light. This happens to create quite usefully distorted or magnified images of the background galaxy. Think of it a little bit like a giant cosmic magnifying glass, and you’ve got the right idea.
Scientists use gravitational lensing to map the distribution of dark matter, as its gravity affects light just like visible matter. For example, the Hubble Space Telescope has captured stunning images of ‘Einstein rings’, where the light from distant galaxies forms perfect circles due to this phenomenon. These kinds of observations have revealed dark matter’s integral role in shaping large-scale cosmic structures, providing indirect but powerful evidence of its existence.

© Adobe Stock; Getty; Alamy; NASA: ESA

Dark energy, the mysterious force accelerating the universe’s expansion, could very
well reshape our entire understanding of physics and cosmology

If dark matter acts as the glue binding the universe together, then dark energy is the mysterious force driving it apart. Together, dark energy constitutes a large percentage of the cosmos, yet its true nature remains one of the greatest enigmas in modern science. Understanding dark energy is critical for unravelling fundamental questions about the universe: its origin, structure and ultimate destiny.
Dark energy was first identified in the late 1990s through observing distant ‘type Ia supernovae’ (‘type one-A’). These exploding stars, often called ‘standard candles’, help astronomers measure distances. Surprisingly, when first discovered, the supernovae appeared dimmer than expected, implying that the universe’s expansion was seemingly accelerating.
This groundbreaking discovery suggested the existence of a previously unknown force acting against gravity. And on something of a cosmic scale. Soon dubbed ‘dark energy’, it’s believed to account for approximately 68% of the universe’s total energy. Though imperceptible on small scales, dark energy drives galaxies apart at ever-increasing speeds, shaping the large-scale behaviour of the universe.
Einstein’s error
Albert Einstein’s general relativity introduced the cosmological constant, representing a uniform energy density filling space. He initially used it to support a static universe, but after Hubble’s discovery of cosmic expansion, he dismissed it as his “biggest blunder”. Decades later, the concept was revived to help explain the accelerating expansion of the universe – restoring validity to Einstein’s abandoned idea. And saving the blushes of one of history’s most famous scientists.
The cosmological constant faces ‘the fine-tuning problem’, as its observed value is extraordinarily small compared to theoretical predictions. This has led lots of scientists to propose alternatives such as the concept of ‘quintessence’, a dynamic energy field that changes over time and space. Other researchers suggest revising our understanding of gravity altogether and exploring modifications to general relativity.
Modern astronomy offers new clues about dark energy. The Dark Energy Survey (DES), completed in 2021, studied over 300 million galaxies using a powerful telescope in Chile. Analysing galaxy distributions and gravitational lensing shed light on how dark energy shapes the universe.
Missions like NASA’s Roman Space Telescope, scheduled for the late 2020s, and the European Space Agency’s Euclid mission, launched in 2023, aim to measure dark energy’s effects with unprecedented precision. These missions could determine whether dark energy’s properties have changed over time, revealing its true nature.
Dark energy is essential for predicting the universe’s ultimate fate. If dark energy grows stronger, the cosmos could – some scientists hypothesise – end in a ‘big rip’, where everything is torn apart as space expands uncontrollably. Alternatively, if dark energy weakens or reverses, gravity might dominate, leading to a ‘big crunch’, a catastrophic universal collapse. Neither sounds like an event to look forward to.
Dark energy’s behaviour could also be unpredictable, leading to scenarios we can’t imagine. Its discovery challenges our understanding of physics and invites new frameworks for interpreting the universe.
Energising research
Dark energy, alongside dark matter, constitutes the majority of the universe. Dark matter interacts with ordinary matter through gravity, while dark energy operates on a different scale, dominating the voids between galaxies. Together, these phenomena challenge the foundations of our understanding of nature’s laws.
The quest to understand dark energy exemplifies humanity’s relentless pursuit of knowledge. Each discovery brings us closer to answering profound questions: where did the universe come from? What governs its evolution? How will it all end? With missions like Roman and Euclid pushing boundaries of what we know, answers may finally be within reach. We live in exciting times.

5%

Visible matter

27%

dark matter

68%

dark energy

Remnants of the supernova
of N103B, Explosions like
this were key in establishing
the idea of dark energy.

Measuring the universe’s expansion

Astronomers measure the universe’s expansion using ‘the Hubble constant’, which does its best to quantify how fast galaxies move away from us. However, different methods produce conflicting results, leading to a major scientific puzzle known as ‘Hubble tension’.
One approach, using Cepheid variable stars and supernovae in nearby galaxies, suggests a higher expansion rate. In contrast, measurements of the cosmic microwave background (CMB) – the afterglow of the Big Bang – indicate a slower rate of universe expansion. This unresolved discrepancy might point to some as-yet unknown physics, such as new particles or a revision of Einstein’s general relativity.
Resolving Hubble Tension is crucial for understanding cosmic expansion and dark energy’s role in shaping the universe. Recent space missions, including the European Space Agency’s Euclid telescope, aim to refine these measurements and bring us closer to solving one of modern astronomy’s greatest mysteries.

Dark energy is tied
to the increasing
rate our universe
is expanding.

© Adobe Stock; Getty; NASA; ESA

Albert Einstein is one
of the key thinkers
who revolutionised our
perception of time.

Join us as we explore time as the fourth
dimension, from Einstein’s relativity to
the block universe theory, uncovering
its mysteries through physics and
human perception

Time is a concept that’s so ingrained in all our lives that we rarely ever stop to question its nature. In part, ironically, because most of us don’t have the time.
We measure it with clocks, schedule our days and weeks around it and feel its effects as we age. And yet, time remains one of the most enigmatic and mind-twisting aspects of existence. What exactly is it? A metaphorical flowing river carrying us through events? A rigid framework within which everything happens? Or is it merely something of an illusion? Yeah, we know: this is a bit of a heavy topic. Maybe the rest of this article is better read with a strong cup of coffee in your hand. Just don’t take too long to make it.
Modern physics endeavours to challenge our intuitive and simplistic understanding of time, presenting a view that is as perplexing as it is fascinating. To grapple with the nature of time, we must explore its inherent duality. There is the human perception of time and how it behaves in the natural, physical world. And they’re two separate things.
Time isn’t just a matter of curiosity for philosophers and physicists, both professional and amateur, it underpins our understanding of reality itself. From the ticking seconds of our daily lives to the vast and unfathomable aeons that have helped shape the cosmos, time – it could be argued – governs all. But does it flow like a river, or is this flow merely an illusion created by our limited perspective? By exploring time through the finite lenses of the human experience, scientific theories and philosophical debates, we can begin to grasp its true essence. And – of course – uncover new questions about its role in the universe.
Relatively speaking
Time to call in a big hitter. Albert Einstein revolutionised our understanding of time with his famous theory of relativity, merging the concept with space into a single entity known as space-time. In this framework, time is not a universal constant but a variable that depends on – and here’s where it begins to get complicated – the observer’s velocity and the presence of gravitational fields. Space-time can be visualised as a malleable fabric, bending under the influence of massive objects such as stars and planets. This curvature, Einstein postulated, affects the passage of time in a phenomenon known as time dilation.
To illustrate this, imagine two twin siblings. One of them stays on Earth while the other is tasked with travelling through space at near-light speed. When the space-faring sibling returns home, they will have aged less than their brother or sister due to the effects of time dilation. Similarly, clocks positioned at higher altitudes, where Earth’s gravitational pull is weaker, tick ever-so-slightly faster than those at sea level. These effects are imperceptible in daily life but critical for technologies like GPS, which must account for such discrepancies to provide accurate positioning data.
Film fans might recognise the concept of time dilation from Christopher Nolan’s 2014 science-fiction epic Interstellar. The effects are exaggerated for Hollywood purposes, but the scientific theory behind it is sound.
This view of time as flexible and relative stands in stark contrast to the way we humans perceive and regard it. We look at it more as a steady, unyielding surge from the past, cutting through the present, moving uncompromisingly into the future. Yet relativity doesn’t just reshape how we understand time, it challenges us to rethink our entire framework of reality. Space-time is not merely a backdrop for events but an active participant in the unfolding of the universe.
Shifting perspectives
For most people, time feels linear and absolute. Days turn into nights, years into decades, and memories fade into history. Our brains create this sense of continuity, linking past experiences to future expectations. This perception is deeply rooted in our biology and, of course, our psychology. For example, studies show that time seems to speed up as we age, possibly because our brains encode fewer new experiences relative to our growing backlog of memories. Possibly because as we age, periods of time become smaller fractions of our entire lives up until that point.
In contrast, physics doesn’t treat time as a one-way street. The equations governing fundamental forces, ones like gravity and electromagnetism, are what’s called ‘time-symmetric’ or ‘T-symmetric’. They work just as well in reverse. This suggests that the distinction between past, present and future may not be fundamental to the universe but something made up by us humans to make sense of our existence. Time, as we perceive it, could just be a convenient illusion that helps us make sense of a reality far more complex than our comparatively modest senses allow us to conceive.
Psychologists and neuroscientists have also explored how cultural factors help shape our experience of time. In Western societies, for example, time is often viewed as a commodity. It’s something to be spent, saved or wasted. Other cultures, however, view time as cyclical or as an intrinsic part of nature’s rhythms. These differences highlight how much of our understanding of time is subjective, even though we often think of it as entirely universal
Here’s a question to ponder: ‘Is time merely an illusion?’ It’s okay. You don’t have to answer straight away. The idea that time might be some sort of myth or artifice has intrigued philosophers and scientists for centuries.
Under the illusion
Physicist and science writer Julian Barbour has long argued that time is not a fundamental property of the universe but something of a more emergent phenomenon. According to Barbour, what we experience as the passage of time is simply a sequence of snapshots, of configurations of the universe existing independently of one another. In this view, our interpretations of the ‘past’ and ‘future’ are illusions created by our brains presenting these configurations in a particular order.
Then there’s the Austrian physicist and philosopher Ernst Mach, who hypothesised that time is nothing but change. “It is utterly beyond our power to measure the changes of things by time,” he wrote. “Quite the contrary, time is an abstraction, at which we arrive using the change of things, made because we are not restricted to any one definite measure, all being interconnected.”
Philosophers like Immanuel Kant have also contemplated the subject, arguing that time is just a construct of the mind, a framework we impose on reality to make sense of our experiences. While physics provides evidence for this view, it raises questions about the nature of existence itself.
Time will tell
The block universe theory takes this concept a little further, proposing that all moments in time – in the past, present and future - exist simultaneously. Also known as ‘eternalism’, it suggests that all events are fixed and unchanging, suggesting that free will may not exist.
This view has some pretty profound implications. If the future is already written, what does that mean for us all as decision-makers? Are our choices preordained, mere illusions of autonomy? While the block universe theory is consistent to a degree with Einstein’s relativity, it’s still quite difficult to reconcile with the way we experience time and life in general.
Critics of the block universe theory argue that it neglects the importance of entropy (the measure of disorder in a system), which provides a clear arrow of time from low entropy (the Big Bang) to high entropy (the depressingly eventual death of the universe). This flow of order to disorder effectively creates the arrow of time, making the future unpredictable and dynamic, which contradicts the fundamental idea of the theory.
Time is a strange thing. It’s both an inescapable aspect of our everyday lives and a deeply mysterious and abstract concept that’s integral to physics. The human perception of time, rooted in memory and expectation, offers us a nicely comforting apparition of order. Meanwhile, physics reveals a universe where time is relative, possibly illusory, and far, stranger than we can comprehend.
Whether time is a fundamental dimension, an emergent property, a convenient illusion or a bizarre abstraction, one thing is certain: our understanding of it is far from complete. As science advances, we may just uncover truths about time that challenge our most basic assumptions about reality. When? Well, all in good time.

Einstein put the idea
forward that mass
warps time in his
theory of relativity.

“the distinction
between past,
present and
future may not
be fundamental
to the universe”

Oldest thing

The

universe

in the

Around 380,000 years after the all-creating Big Bang, the universe underwent a dramatic shift. The scorching plasma that had filled space finally began cooling just enough for atoms to start forming, allowing light to travel freely for the first time. This now-ancient glow, known to astronomers as the ‘cosmic microwave background’ (understandably shortened to ‘CMB’), has now been travelling through space for over 13.8 billion years.
The CMB is faint microwave radiation, invisible to the naked eye but detectable with particularly sensitive instruments. It appears as a map of subtle temperature variations across the sky, offering a snapshot of the universe at its earliest stages.
These variations hold vital clues about how the first galaxies and stars began to form. The CMB is a relic from the universe’s infancy, one which – like some sort of ledger – carries a record of its origins. It also offers stargazers an invaluable window into the birth and evolution of the cosmos.
So, if you want to see the oldest thing in the universe, keep your eyes peeled for the light that emanates from the cosmic microwave background. Provided, that is, you have access to some of the world’s strongest scientific equipment.

The cosmic microwave
background imaged by the
ESA’s Planck Space Telescope.

This nail-biting scene in the
film Interstellar takes place
on a planet orbiting a black
hole and involves time dilation
where people experience
time at different rates

Our perception of
linear time could be
different to how the
force actually behaves.

Our understanding
is that black holes
severely warp time due
to their gravitational fields.

© Adobe Stock; ESA; Alamy

Unravel the mysteries surrounding our
local stellar neighbourhood

46

28 how was the moon formed?
34 planet nine
40 Mysteries of Mars
46 can saturn’s moons sustain life?
50 the oort cloud
54 Will humans set foot
on another planet?

50

How Was The Moon Formed?

The Moon is
essential to Earth,
but its origins are
fiercely debated.
Let’s explore some
key theories about
its formation,
including the giant
impact hypothesis
and a few intriguing
others…

For as long as humans have gazed up at the night sky, the Moon has been a source of wonder to us. Our ever-present celestial companion exerts an undeniably profound influence on Earth, stabilising our planet’s tilt, driving the tides of our seas and oceans, and inspiring centuries of mythology and science. Yet, despite its familiarity, the Moon’s origins still remain one of the most fascinating puzzles in planetary science.
For centuries, researchers have sought to answer a seemingly simple question: ‘How did the Moon come to be?’ Theories aren’t exactly in short supply, with each offering a unique perspective on the Moon’s formation. Among them, the ‘giant impact hypothesis’ stands out as the most widely accepted - and realistic - explanation. Though significant mysteries still surround this theory. Alongside it are alternative ideas that paint intriguing, if somewhat less likely, scenarios. By trying to unravel the Moon’s origin story, we can not only learn about our nearest celestial neighbour but also gain plenty of insights into our planet and the chaotic infancy of our solar system.
As we just touched upon, the most compelling theory for the Moon’s formation is the giant impact hypothesis. According to this scenario, approximately 4.5 billion years ago, a Mars-sized ancient planet named Theia collided with a young Earth. The impact was cataclysmic, ejecting vast amounts of debris into space. Over time, the debris orbiting Earth coalesced to form the Moon we know today.
The theory is supported by fairly compelling evidence. Lunar rocks brought back by the Apollo missions revealed a striking similarity to Earth’s composition, particularly when it comes to isotopic signatures of oxygen. This suggests that the Moon was born from Earth’s mantle material rather than being a foreign body captured by Earth’s gravity.
However, the giant impact hypothesis is not without its challenges. One question is why the Moon’s isotopic composition is similar to Earth’s, rather than showing a mix of Earth and Theia’s materials. Recent studies suggest that Theia might have been compositionally alike to Earth, though this raises its own set of unanswered questions and riddles. Plus, the Moon’s lack of volatile elements - those that evaporate at low temperatures (like sodium, zinc or potassium) - remains an area of ongoing research, potentially linked to the intense heat of the impact.
While the giant impact hypothesis dominates scientific discourse on this subject, plenty of other ideas and arguments offer alternative explanations for the Moon’s origin. Some are reasonably convincing if you listen to them with an open mind. Others? Well, they’re slightly less persuasive.
Hybrid theory
Next, we have Edouard Roche’s ‘co-accretion theory’, which suggests that the Earth and the Moon formed together from the same ‘protoplanetary disk’. In this scenario, both bodies would have grown simultaneously from a shared reservoir of gas and dust. However, this theory struggles to explain the differences in density and composition between the two bodies.
For instance, the Moon’s core is significantly smaller and less dense than Earth’s, which would be pretty unlikely had they both formed from the same materials simultaneously and in close proximity. Plus, the isotopic similarity between the two bodies raises hefty questions about why they aren’t identical in other chemical properties. It’s a reasonable hypothesis, but not one which stands up to too much scrutiny, unfortunately.
Then there’s the ‘capture theory’, which posits that the Moon was an independent object, perhaps something like a wandering asteroid or a minor planet, that was captured by the draw of Earth’s gravity. This idea, while fascinating, requires some extraordinarily precise conditions. A captured Moon would likely have an eccentric orbit, so the Moon’s nearly circular orbit around Earth contradicts this idea.
Also making this scenario unlikely is the isotopic composition of lunar rocks, given that they’re so strikingly similar to Earth’s. If the Moon originated elsewhere in the solar system, its isotopic signatures would likely differ significantly from Earth’s.
Another hypothesis is the fission model, which proposes that a rapidly spinning Earth flung off a chunk of its mantle, eventually whittling down to form the Moon. This idea dates back to the 19th century and is often associated with George Darwin, son of the famous evolutionary scientist Charles Darwin.
The fission model imagines a scenario where the young Earth’s rapid rotation caused a portion of its mantle to separate, perhaps due to centrifugal forces. While this theory explains the isotopic similarity between Earth and the Moon, it also has some significant flaws. For one, the current angular momentum of the Earth-Moon system doesn’t align all that well with what the model predicts. Plus, modern understanding of planetary formation suggests such an event is highly improbable. Weirder things have happened, but it just doesn’t seem all that likely to those in the know.
Making an impact
The ‘synestia hypothesis’ is another intriguing concept. Albeit also a fairly complex one. A synestia is a hypothetical, doughnut-shaped structure of vaporised rock that gets formed after a giant collision. In this theory, both Earth and the Moon emerged from the same synestia, sharing materials and isotopic characteristics. This idea is still under exploration and isn’t fully formed as yet, but it offers a compelling way to address some of the challenges faced by the giant impact hypothesis or any of the other ideas that exist but also aren’t 100% convincing.
A more recent addition to the conversation is the ‘multiple impacts hypothesis’, which suggests that the Moon was formed from debris produced by several smaller impacts rather than one giant collision. This idea could potentially account for some of the compositional anomalies observed in lunar rocks. However, it introduces a fair amount of complexity to the narrative and – like other as-yet-unproven theories – requires further evidence in order to gain wider acceptance.
Despite significant advances in technology and decades of serious study, the Moon’s formation story is far from complete. One lingering mystery is the similarity in isotopic composition between Earth and the Moon. Did Theia share a common origin with Earth, or did the violent impact somehow homogenise the materials? Understanding this isotopic similarity could unlock new insights into the dynamics of early solar system collisions.
Another question involves the Moon’s internal structure. While seismic data from lunar missions suggest the presence of a partially molten core, the exact nature of the Moon’s interior remains elusive to us. Could hidden reservoirs of water or other volatiles exist deep beneath its surface? If so, how did they survive the intense heat of the Moon’s formation? One day, hopefully, we’ll find out.
A problem like maria
The Moon’s far side, with its rugged terrain and lack of maria (the dark plains visible on the near side), also raises questions. Why are the two hemispheres so vastly different? Theories range from variations in heat distribution during its early formation to the lasting effects of another massive collision. Recent lunar missions have provided detailed maps of the far side, revealing clues but not definitive answers.
The Apollo missions and subsequent unmanned explorers sent up from Earth since Apollo have provided invaluable samples of lunar material. These rocks and surface samples offer a window into the Moon’s history, preserving evidence of ancient volcanic activity, meteoroid impacts and solar wind exposure. By studying these samples, scientists have pieced together a timeline of the Moon’s geological evolution, from its fiery beginnings to its current almost dormant state.
One of the most significant lunar discoveries ever made is the discovery of the presence of tiny amounts of water in lunar rocks. While the Moon is mostly dry, these traces suggest water may have been delivered by comets or retained from the Earth’s mantle material during the giant impact. Understanding the origin and distribution of this water could have weighty implications for future lunar exploration and even for theories about the origin of water on Earth.
Beyond its creation, the Moon continues to play a vital role in life on Earth. Its gravitational pull stabilises our planet’s axial tilt, creating a steady and reliable climate that’s conducive to life. The tides it drives have shaped ecosystems and inspired countless scientific discoveries, too. For instance, tidal forces may have played a role in the emergence of early life by creating dynamic coastal environments.
The Moon’s influence extends to human culture as well. From ancient calendars to modern scientific research, its cycles have been a constant presence in our understanding of time and nature. Full moons and lunar eclipses, in particular, have fascinated humanity, serving as both omens and opportunities for scientific observation.
Over the Moon
Future lunar exploration promises to shed new light on the Moon’s formation and history. NASA’s Artemis program aims to return humans to the lunar surface, with plans for sustainable exploration and maybe even a lunar base of some kind. These missions will provide unprecedented access to the Moon’s surface and subsurface, potentially answering some of the most perplexing questions about its origin.
One of the mission’s goals is to explore the Moon’s south pole, where permanently shadowed regions may contain water ice. This resource could be critical for future human missions, providing drinking water, oxygen and even (potentially) fuel for spacecraft. By studying these icy deposits, scientists hope to understand their origin and what they reveal about the Moon’s early environment.
International collaborations are also playing a significant role. Countries like China, India and Russia are conducting their own lunar missions, contributing to the global collective knowledge of the natural satellite. Robotic landers and orbiters continue to map the Moon’s surface, study its composition and test technologies for future exploration.
Studying the Moon offers a window into the broader processes that govern planetary systems. Its (fairly) pristine surface preserves a record of early solar system history, including the period of heavy bombardment when countless asteroids and comets shaped the planets. By understanding our closest neighbour, we take one step closer to deciphering the history of the cosmos – as well as Earth – and how it came to be.
The Moon’s formation also has implications for exoplanetary science (the study of other solar systems). By studying how the Earth-Moon system formed and evolved, scientists can refine models of planetary formation in other star systems. This knowledge could help identify exoplanets with similar characteristics to our own, improving our understanding of other potentially habitable worlds.
The Moon remains a giant source of mysteries. Why does it lack a significant magnetic field? Could its early history include a magnetic dynamo similar to Earth’s? And what accounts for the strange ‘moonquakes’ detected by seismometers during the Apollo missions?
The story of the Moon’s formation is likely one of spectacular cosmic violence and intricate and complex physics. From the giant impact hypothesis to alternative theories, each perspective adds a piece to the puzzle. As we continue to explore and study the Moon, we uncover not only its secrets but also the history of our planet and the solar system as a whole.
By understanding the Moon, we gain a deeper appreciation of our place in the universe and the forces that shaped our world. One thing we do know for certain is that it’s not made of cheese.

© Adobe Stock

“Lunar rocks
brought back by
the Apollo
missions were
similar to earth”

The solid iron core
of the Moon has a
temperature that clocks
in at over 1,300°C.

This graphic depicting
the giant impact theory
shows how the Earth
and Moon could have
formed from a collision.

Moon

mysteries

Unanswered questions  about our lunar neighbour

Why does the Moon have far fewer
volatile elements than Earth? Scientists suspect that the intense heat created during the Moon’s formation may have vaporised them all. However, the exact processes and accuracy of this idea are uncertain.
What caused the dark ‘maria’
on its surface?
These ‘basaltic plains’ are thought to be ancient lava flows, but why they are concentrated on the near side and largely absent on the far side is still unknown.
Could there be water reserves
deep within the Moon?
Recent studies suggest small amounts of water might exist in lunar minerals, potentially trapped during its violent formation or delivered there by comets.
How accurate is our understanding of
lunar seismic activity?
Apollo-era seismometers detected what they referred to as ‘moonquakes’, but the causes - whether tectonic, meteoric or thermal - remain a fairly sizeable mystery requiring modern instrumentation.
Did the Moon have a magnetic field?
Evidence in lunar rocks hints at a magnetic field billions of years ago, but its origin and duration are still unresolved questions for scientists.
Why is the far side of the Moon
so different?
The stark contrast could be due to variations in crust thickness or the effects of colossal impacts early in its history. Or because of something else astronomers haven’t yet considered.

There are a few
theories that propose
the Earth and Moon
formed together.

Space Oddity:

Apollo 10’s music mystery

During Apollo 10’s 1969 mission, astronauts orbiting the Moon experienced an eerie, unexplained ‘whistling sound’ through their communication headsets while passing over the Moon’s far side, where radio contact with Earth was impossible. This peculiar noise likened to ‘outer-space-type music’, lasted nearly an hour, sparking intrigue and speculation. Was it alien contact? Or merely some kind of computational glitch?
Decades later, NASA clarified the source: the sound was, they claim, likely caused by radio interference between the spacecraft’s VHF radios, which were transmitting data between the lunar and command modules. While technically explainable, the incident highlights how even seasoned astronauts can be unsettled by the unexpected in deep space.
The ‘Moon music’ became a cultural curiosity, emblematic of humanity’s smallness and vulnerability in the cosmos. It also reminds us how easily perception and mystery can transform a routine technical anomaly into a story that stirs the imagination.

Future manned
missions to the
Moon will reveal
more of its secrets.

Our Moon has two
very distinct sides,
and we don’t fully
know why.

© Adobe Stock; NASA; Getty

It’s never been directly observed but
astronomers believe there’s a ninth planet
in the Solar System – and it’s still not Pluto!

Written by David Crookes

Before 2006, astronomers generally agreed that there were nine planets in our solar system. There was Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto – an order that had seemingly been set in stone in 1930 when Clyde W Tombaugh discovered Pluto at the Lowell Observatory.
But then the International Astronomical Union (IAU) dropped a bombshell. For several years, scientists began to realise that Pluto, which resides in the Kuiper Belt – a zone situated beyond Neptune’s orbit packed with icy bodies – wasn’t unique. Other objects, such as Eris and Makemake, were extremely similar to Pluto, prompting a major rethink.
The IAU announced an official definition of a planet. It said a celestial body needed to orbit around the Sun, have sufficient mass to form a near-round shape and clear the neighbourhood around its orbit. Pluto fit the description for two of those three terms, but it faltered with the third. Since it hadn’t become gravitationally dominant and shared an orbital space with other similar-sized objects, it didn’t clear the neighbourhood, and so Pluto was reclassified as a dwarf planet.
Even so, mounting evidence suggests that the solar system still has nine planets. The trouble is, it’s not Pluto, and no one has ever directly observed the hypothetical ninth member potentially lurking at the far reaches of the solar system well beyond Neptune. Instead, various calculated models point to its existence in order to explain why various parts of the solar system behave in the way they do, and it certainly makes for a compelling case for an extra celestial body to be thrown into the mix.
One of the key puzzles the existence of Planet Nine could help explain is why some dwarf planets and small icy objects follow orbits that cluster together beyond Neptune, and this was the catalyst for proposing the extra planet.
the pluto killer
The hypothesis for Planet Nine emerged almost a decade ago, put forward by Konstantin Batygin, an astronomer at the California Institute Of Technology (Caltech) and his colleague Mike Brown, but their involvement in the search for Planet Nine is intriguing. One of them – Brown – was instrumental in Pluto’s demotion to dwarf planet status, and he celebrates the fact to some extent, albeit with great humour. His username on the social networking service Bluesky is @plutokiller. That Brown is on the hunt for Pluto’s replacement is akin to hammering the final nail in Pluto’s planetary coffin.
“Our interest was sparked by the clustering of arguments of perihelion noted by Trujillo and Sheppard in 2014,” says Batygin, referring to research which showed extreme trans-Neptunian objects (TNOs) clustering with similar orbital angles – they are tilted about 20 degrees with respect to the plane within which the planets orbit the Sun. “While incomplete at the time, it provided an intriguing hint of a larger dynamical structure in the outer solar system.”
The 2014 study highlighted the eccentric orbit of a Kuiper Belt object called 2012 VP113, which was very similar to that of a large object called Sedna, a dwarf planet co-discovered by Brown in November 2003. Sedna orbits the Sun well beyond the orbit of Neptune – it is, in fact, 90 times farther away from the Sun than the Earth, which makes it the most distant known world in the solar system. It also follows a highly elliptical path around the Sun, which takes more than 10,000 years.
Hidden influence
With hints that more eccentric TNOs were waiting to be found, Brown and Batygin got to work on a potential explanation. They eventually suggested that an extra planet could be directly influencing the behaviour of the TNOs. If Sedna had a close gravitational encounter with Planet Nine around the time the solar system was being formed, then this could explain both Sedna’s mysterious orbit and why it never gets very close to Neptune, unlike the more standard Kuiper Belt objects.
Indeed, a ninth planet, Brown and Batygin said, could have slowly pulled Sedna and 2012 VP113 into orbits less connected to Neptune. For this to happen, the ninth planet would, they added, need to have a mass about ten times that of Earth and orbit 20 times farther from the Sun than Neptune. It seemed plausible, particularly since other explanations were tried to no avail. Put a Planet Nine in there, and the TNOs behaved as observed. Take it out, and they didn’t.
When four more TNOs with similar mismatched orbits were found, this seemed to lend even further weight to the argument. Brown and Batygin said such orbital oddities could be down to sheer chance, and that is very strongly pointed to the existence of a ninth planet. Since then, even more eccentric TNOs have been found. Again, a ninth planet seems to be a good explanation for why the solar system hosts a distant population of highly inclined trans-Neptunian bodies.
Planet Nine, according to NASA, would also impact other understandings of the solar system, such as the existence of objects that reside between the giant planets and orbit the Sun in a retrograde direction. What’s more, Planet Nine would also answer another perplexing puzzle: why our solar system lacks a super-Earth when such planets are common around other stars. If Planet Nine is a super-Earth, then that’s another case closed.
Of course, there is every chance that those behaviours can be explained in different ways and that Planet Nine simply does not exist. Some possibilities include an invisible giant disc of dust or a baby black hole. But while there are sceptics and there is much controversy over the whole issue of a ninth planet, the debate continues. In the same way, there is no definitive proof that the additional solar system planet exists, and there is no definitive proof that it doesn’t either.
Today, Brown and Batygin say there are five key lines of evidence in support of Planet Nine – up from the four proposed in 2016. “These are the alignment of stable TNO orbits, the clustering of their orbital planes, the creation of highly inclined long-period scattered objects, retrograde Centaurs in the outer solar system (Centaurs being small icy objects orbiting the Sun between Jupiter and Neptune), and a flux of long-period Neptune-crossing TNOs,” highlights Batygin.
The latter was added in 2024, and it relates to Brown and Batygin’s study – in conjunction with Alessandro Morbidelli of the Observatoire De La Côte d’Azur in Nice, France, and David Nesvorny of the Southwest Research Institute in Boulder, Colorado – which found that Planet Nine could be used to explain why some TNOs are found closer to the Sun than expected.
Simulations considered the possibility that the galactic tide caused by gravitational forces within the Milky Way or passing stars could be affecting these oddly positioned TNOs, but only when Planet Nine was factored in would the simulations align with the observed population of 17 TNOs that cross Neptune’s orbit. A Planet Nine, they concluded, was pulling the orbits of the TNOs closer to the Sun. They showed that the gravitational influence of Planet Nine was needed to create and sustain the flux of these particular TNOs.
Given its potential influence, however, it’s perhaps surprising that Planet Nine has never been observed, even though astronomers have seen many exoplanets in other galaxies. But there are many potential reasons for this. The first is the likely size of Planet Nine. “Our models suggest that to explain the data, the mass of Planet Nine has to clock in at approximately five Earth masses – consistent with a super-Earth-type object,” says Batygin. That makes it rather small and more difficult to spot. Space is also rather packed, which can make it difficult to identify something like a planet amid a sea of bright, moving objects.
Needle in a haystack
But there are other factors at play, too. “Planet Nine is extremely faint, likely at magnitude 24 or dimmer, and its exact position in the sky is unknown, making it hard to detect despite knowing its orbital parameters,” Batygin adds. Yet hope is on the horizon, and it may not actually be long before the mystery of Planet Nine is cleared up one way or the other. There’s certainly an appetite for it, with even a citizen science project called Backyard Worlds: Planet Nine hoping to find it using data from NASA’s Wide-field Infrared Survey Explorer mission. Who knows, you may be the one to spot it.
In any case, Batygin, Brown and many other astronomers are hoping that the Vera Rubin Observatory in Chile will finally make a breakthrough in the hunt for Planet Nine. This novel wide-field telescope built on the summit of Cerro Pachón comes into operation in July 2025. Its 8.4-metre telescope, 3,200-megapixel camera and automated data processing system will be powerful enough to scan the entire sky within a few days, and its ability to map small objects is sure to be invaluable.
Searching the skies
“The Vera Rubin Observatory will provide an unprecedented ability to survey the sky for faint, distant objects, greatly enhancing the search for Planet Nine,” Batygin confirms. If it does, it will have succeeded, where other methods have failed. A search in December 2021 using the Atacama Cosmology Telescope in Chile failed to make a breakthrough, and, in March 2024, data from the Panoramic Survey Telescope and Rapid Response System (otherwise known as Pan-STARRS) in Hawaii also failed to show any sign of the celestial body.
In that instance, researchers were able to look more closely at a region of sky where Planet Nine is thought to be located but, although they came away disappointed, it has at least succeeded in narrowing down its potential location. Cumulatively, the different sky surveys have now excluded 78% of the search space, but that means there’s 22% of the vast sky to go.
It is why astronomers continue to search and why they have to go on the assumption that it’s there until it’s not. Indeed, that’s the line being taken by Brown, who wrote in a blog post back in 2021, “So far, we have not found a reason to discard the Planet Nine hypothesis. Will one someday? Perhaps. If the evidence for the existence of Planet Nine were to unravel, we would be sad, but we would have to give up the idea. We have been prepared for that moment since the day we first proposed Planet Nine.”
If it is found, however, then it still won’t be the end of this mystery. Should its existence be confirmed, astronomers will spend years trying to uncover its secrets and work out how it is likely to have formed. Discovering a ninth planet hidden in the far reaches of the solar system would be a big deal, and it could turn our understanding of how the solar system formed and evolved entirely on its head.
Already, some theories are being put forward as to how Planet Nine could have been born. “Planet Nine may have formed alongside Uranus and Neptune but was scattered out by Jupiter or Saturn,” Batygin offers. “Its orbit was then stabilised in the solar system’s early days due to interactions with a birth cluster of stars.”
Astronomers will also want to know whether the planet could have many moons or whether it’s actually a rogue planet – one which was captured by our Sun after it was flung away from its own star system, left to wreak havoc in the outer solar system. The first step, however, is actually finding it, and you can bet Brown and Batygin will be shouting from the rooftops if it finally comes into view.

The telescope in the
Lowell Observatory
where Clyde Tombaugh
discovered Pluto.

Konstantin Batygin
(left) and Mike Brown
(right) are searching
for Planet Nine.

“a ninth planet
could have pulled
Sedna and 2012
VP113 into orbits
less connected to
Neptune”

Brown and Batygin
propose Sedna’s odd orbit
is a result of interactions
with Planet Nine.

Planet Nine

The Vera C Rubin
Observatory in Chile
might hold the key to
discovering Planet Nine.

The continued fight for

Pluto

Although Pluto is officially classified as a dwarf planet, not everyone accepts the definition. Among the planetary scientists lending their backing to Pluto is Alan Stern, the principal investigator behind New Horizons, which was the first mission to the frozen faraway dwarf planet and the Kuiper Belt.
Along with many other planetary scientists, he rejects the decision to make Pluto a dwarf planet, as voted for by members of the International Astronomical Union just nine months after New Horizons launched in 2006.
He says it doesn’t make scientific sense and argues that the definition of a planet was created by astronomers who study stars, galaxies and black holes rather than planetary scientists and is poorly worded as a result. In fact, planetary scientists would go as far as classifying all dwarf planets as planets rather than make a distinction. If this is the case, then Planet Nine would certainly need a different number (although, if found, it would be renamed anyway).

© Adobe Stock; Getty; Wikipedia Commons / Christopher Michel

Planet Nine would
neatly explain
eccentric orbit patterns
out in the Kuiper Belt.

“the mass of
Planet Nine has
to clock in at
approximately
five Earth
masses”

© NASA; ESA; Adobe Stock; Alamy; Getty

The Kuiper Belt lies
beyond Neptune and
is made up of
countless icy bodies.

Planet Nine, if it does
exist, lies incredibly far
out on the fringes of
the solar system.

just

How big

is

planet Nine?

The potential size of Planet Nine has altered over the years as different simulation models shed more light on this theoretical world. For a while, the estimate, based on the apparent force of its gravitational pull, was that Planet Nine is between five and ten times the mass of Earth, But more recent studies by Mike Brown and Konstantin Batygin pinpoint it at about 6.2 Earth masses.
If this is the case, then it would put Planet Nine ahead of all of the terrestrial planets – that is, Mercury, Venus, Earth and Mars, placing it fifth behind Jupiter (the largest), Saturn, Uranus and Neptune. Its composition is also most likely to be gaseous, like Neptune, with a rocky core, although it could also be a rocky super-Earth. It is also likely to have a diameter two times the width of Earth.

Some major questions abound about
the Red Planet and how it ended
up so desolate. Can we work
out what happened to this once
potentially habitable world?

© Adobe Stock; Getty; ESO

Written by Jonathan O’Callaghan

Of all the planets of our solar system, perhaps none are as alluring and confusing as our neighbouring world, Mars. Located about 100 million kilometres (60 million miles) further from the Sun than Earth and half our planet’s size, Mars is a rocky world that looks like it once had water on its surface, with valleys and dents that appear to have been formed by lakes, seas and rivers.
Yet today, Mars is barren. Its atmosphere is mostly carbon dioxide, and the thinness of the atmosphere means that any liquid on its surface quickly evaporates into space. At its poles, there are vast expanses of ice, hints of its watery past, while beneath its surface, there might still be liquid water trapped today.
With water, Mars might have once played host to life, most likely microbial life forms during the brief window the planet was habitable, so understanding how Mars lost its water is crucial in our broader search for life in the universe. It is possible – if Mars did host life – there could be remnants of this life today. Ongoing missions are hoping to answer this question.
It is not just the potential habitability of Mars that is intriguing. The planet must have had a magnetic field in the past to retain an atmosphere and enable liquid water to exist. What happened to this magnetic field, and is losing it a recurrent process on rocky planets? Meanwhile, its two moons, Phobos and Deimos, are unusual in being extremely small. Where did they come from, and what do they tell us about moon formation?
Ultimately, our biggest question about Mars is whether the planet might still be habitable today, not just to microbes but perhaps to humans too. If we land humans on the Martian surface, is there any hope for long-term survivability, turning humanity into a multi-planet species?

“Mars might
have once played
host to life, most
likely microbial
life forms”

mysteries of Mars

What happened to

Mars’s water?

Arguably, our most important question about Mars is: what happened to its water? To understand what might have taken place, we first need to delve into the planet’s history.
Evidence suggests that sometime up to 4.1 billion years ago, water might have been present on Mars. That’s based on simulations of the planet, which point towards a large ocean existing in the planet’s northern hemisphere, which we can possibly see remnants of today in the form of a shoreline. More locally, there appear to have been lakes and seas on the surface. NASA’s Curiosity and Perseverance rovers are currently exploring two of these ancient lakes – inside Gale Crater and Jezero Crater, respectively.
If there was water, there could have been life. Most likely, this would have been simple microbial life, with not enough time for it to evolve into more complex forms, such as on Earth, because, around 3 billion years ago, this water seems to have disappeared. We can see the remains of this water in the form of salty deposits on the surface, while there also appear to be reservoirs of water still buried deep under the Martian surface, several kilometres underground.
We think Mars lost its water because, around this time, it also lost its atmosphere. The low pressure and chilly temperature on the surface of Mars resulting from this loss would have meant that any liquid would either instantly evaporate or freeze; the oceans were lost to the sky. Water that was evaporated into the atmosphere as vapour would then have been blown away by the Sun into space. Eventually, all the water was gone, leaving behind the barren red world we see today.
It’s somewhat strange to think of Mars as a once-watery world, perhaps on its way to becoming a habitable oasis like Earth before something happened. The answer to this mystery may be tied to another crucial question about the planet’s core.

There is evidence
pointing towards
Mars being once
flush with water.

Without a magnetic
field, solar winds would
have devastated Mars’s
atmosphere.

The planet’s core likely
powered a protective
magnetic field 4 billion
years ago.

Why did

Mars

lose

its

magnetic field?

The loss of water and the atmosphere on Mars is almost certainly linked to the planet’s interior. At some point early in its history, Mars’s core likely drove a large magnetic field, like on Earth, which trapped an atmosphere around the planet. That was until something caused this dynamo to stop, and the path to a desolate planet began. So what happened?
We can see evidence of an ancient magnetic field on Mars today. Orbiting spacecraft have observed that some locations on the surface appear to have magnetic hot spots, with the planet’s lost magnetism remaining trapped in some rocks on the surface. These observations suggest a magnetic field existed at least 4 billion years ago.
More recent evidence for a Martian magnetic field comes from a Martian meteorite called Allan Hills 84001, which was discovered in Antarctica in 1984. Once claimed to have signs of bacterial life inside it – now debunked – the meteorite does seem to have stored magnetism from a Martian magnetic field about 3.9 billion years ago, evidence that the magnetic field existed for some time, allowing water to exist.
The magnetic field on Mars – like on Earth – was probably the result of a hot outer liquid iron core surrounding a solid inner core inside the planet, something NASA’s InSight lander studied. As material moved between the inner and outer core, it caused convection, driving a dynamo process that created a powerful magnetic field encompassing the planet. That was until something mysterious seemed to have stopped this process, possibly the result of the core separating into layers of iron and hydrogen, ending convection, although scientists are not sure.
Earth’s magnetic field protects our planet against incoming charged particles from the Sun on the solar wind. These particles would blow away our atmosphere, but instead, they are funnelled to the poles by our magnetic field. Sadly, in Mars’s case, without a magnetic field, the planet no longer had such protection. The solar wind stripped away its atmosphere, its water evaporated, and the rest is history.

“some locations
on the surface
appear to have
magnetic
hot spots”

NASA’s MAVEN orbiter
mapped electric
currents around Mars
that are fundamental to
its loss of atmosphere.

© Adobe Stock; Getty;

Are

phobos

And

deimos

captured asteroids?

The two moons of Mars are peculiar – called Phobos and Deimos, they are both remarkably miniature and orbit very close to the planet. That has led scientists to wonder if they might be captured objects from elsewhere in the solar system that ended up orbiting Mars – but they might not do so forever.
Phobos is just 22 kilometres (14 miles) across, and Deimos is only 13 kilometres (eight miles), each small enough to comfortably fit inside Greater London. They also orbit Mars very closely – with Phobos just 6,000 kilometres (3,700 miles) away and Deimos further at 23,500 kilometres (14,600 miles) – completing an orbit every eight hours and 30 hours, respectively. By comparison, our Moon is some 3,500 kilometres (2,200 miles) across and about 385,000 kilometres (240,000 miles) from Earth. Phobos is also getting two meters (six feet) closer to Mars every century – and it will either crash into the planet or break apart into a ring in 50 million years.
Phobos and Deimos are both rocky, with the former having a large crater called Stickney that covers much of its surface, although both are more broadly marked by craters. They are also dark in appearance, with ice mixed into their rock. That makes them look quite similar to asteroids we see drifting out in the solar system, suggesting they might have been such objects that just happened to get snared by the planet’s gravity billions of years ago.
Not everyone is so sure. Some research suggests the two objects are actually more similar to the composition of Mars, which could mean they are actually fragments of the planet itself – perhaps thrown out by something smashing into the surface long ago. Other research suggests they are more similar to comets than asteroids, meaning they could be icy objects instead that were grabbed by the Red Planet.
We might find out the answer to this mystery in the coming years when a Japanese spacecraft – the Martian Moons Exploration (MMX) mission – travels to Phobos. It will attempt to scoop pieces of the satellite and return them to Earth by 2031, perhaps telling us, once and for all, how Mars got its moons.

Phobos and Deimos
could be captured
asteroids or debris left
after an impact.

why is

split in two?

One of the strangest things about Mars is that it seems to be divided into two halves. The planet’s northern hemisphere is a few kilometres lower than the southern hemisphere for reasons we can’t explain.
This divide between the north and south of the planet has been imaged by spacecraft such as ESA’s Mars Express orbiter. The north of Mars is also much smoother than the south, with fewer craters, which might be a result of more volcanic activity in the north covering more of its craters. This would be evidence of past plate tectonic activity on the planet, a phenomenon on Earth that produces volcanoes, which is not present on Mars today.
The divide between the north and south suggests there might have been a significant impact on Mars long ago, which either created a massive dent in the northern hemisphere or produced enough lava in the south to form the planet’s highlands. If the two halves of Mars are the result of an impact, it might be one of the most remarkable known events in the solar system – creating an explosion equivalent to some 100 billion gigatonnes of TNT and producing the largest known crater on any planet.
The lower elevation of the northern hemisphere might have meant it was more suitable for a large ocean to form as well, with some parts of this hemisphere appearing like the remnants of an ancient shoreline from this ocean. Today, the southern hemisphere is more susceptible to dust storms because Mars is closer to the Sun during summer. The heat from our nearest star causes dust to rise from the surface, causing these storms, which can encompass the entire planet and block the Sun’s light – a problem encountered by fictional explorer Mark Watney in the book and film The Martian.

Topographic image
of Mars showing
the difference in
hemispheres.

Mars as seen by
Viking 1 in 1976,
showing large
impact craters.

life on Mars?

is there

If Mars was once wet, that, of course, begs the ultimate question: was there life on Mars, and could there still be today? For years, scientists have sought an answer to this mystery, and today, we are closer than ever to finding out.
On Earth, we know that for life to survive, it needs heat, water and a source of energy. On ancient Mars, it appears that the planet had all three of these ingredients. Rovers on Mars have studied rocks on the surface that show signs of organics, the building blocks of life, while we know there was almost certainly running water at one point and radiation from the Sun to provide energy. That means either Mars had all the right conditions yet no life formed, an interesting result in itself in our broader search for life in the universe, suggesting that not all wet rocky planets become habitable, or it did have a simple form of life, the first known second genesis of life on another world.
If life did exist on Mars, it might have been some form of methanogens, methane-producing microbes that could have produced some of the methane we see on Mars today. Evidence for this life might still be trapped in rocks on Mars; methanogens might have altered the chemistry of some rocks, while there could even be fossilised microbes inside some rocks awaiting discovery. Some life might still exist today inside pockets of water trapped under the Martian surface, reservoirs of habitability where there is just enough heat and water to provide conditions for simple life to survive.
A NASA rover, called Perseverance, is collecting samples on Mars that might contain evidence of past Martian life. It is looking for rocks in an ancient river delta that might have had the right conditions to store life within them. A future mission, part of NASA’s multi-billion-dollar Mars Sample Return program, will then collect these samples and launch them back to Earth, where scientists can study them in earnest in large laboratories and hopefully tell us if life ever existed on Mars or even still clings on today.

Martian rovers are
exploring the planet
surface with the hope
to find signs of life.

© Adobe Stock; Getty; NASA; SpaceX; AppleTV/For All Mankind

“If life did exist
on Mars, it
might have been
some form of
methanogens”

The TV Show For All
Mankind envisions
what life on Mars could
realistically look like.

could we

live on Mars?

When you think of your next holiday destination, Mars might not seem like an obvious choice. Without a spacesuit, you’d boil alive on its surface because of the low pressure – if you didn’t freeze to death first. So why would we want to go there, and could we survive there at all?
Getting humans to Mars is one of the significant steps in space exploration after we return to the Moon. With a journey to the planet taking about eight months and a rocky surface that, unlike Venus, wouldn’t kill or crush you instantly with the right equipment, it does have its positives. Humans on Mars could set up settlements for scientific study and probe some of the mysteries of the planet in a way rovers cannot match.
Some rockets being developed at the moment, like SpaceX’s Starship rocket, are designed with Mars settlement in mind. In theory, it should be possible for us to live on Mars in similar habitats that have been used on the Moon. Using pressurised structures on the surface, astronauts could live on Mars for relatively long periods, venturing outside in spacesuits when necessary. There are also proposals to use inflatable habitats or even to dig underground to provide more liveable space for people on Mars.
We could also turn the carbon dioxide on Mars into oxygen, something tested by an instrument on board NASA’s Perseverance rover called MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment). This means that humans on Mars could take steps towards becoming self-sustaining without having to rely on resupplies. Far in the future, it might be possible to terraform Mars, turning carbon dioxide into oxygen and making the planet similar to Earth. While science fiction for some, living on Mars might be something we one day accomplish.

Humans could harness
the resources on
Mars to become
self-sustaining.

© NASA; Adobe Stock; Getty;

Can Saturn’s Moons Sustain Life?

The Saturnian moons
Enceladus and Titan
are some of most
intriguing candidates
for habitability in the
solar system

If you wanted to find other life in the solar system, Saturn might not seem like the evident first port of call. Mars, with its suspected once-wet surface, looks quite tantalising, as does Venus, a rocky world similar in size to our own that somehow turned into a veritable hellscape.
But make the journey 1.4 billion kilometres (900 million miles) from the Sun, and you might be pleasantly surprised. Here, orbiting Saturn, you will discover two moons that might be among our best candidates for finding alien life. These are the moons Enceladus and Titan, and for different reasons, they are both of our top targets to answer the question of whether we are alone in the universe.
Water world
Saturn was visited by several spacecraft, but most notably by NASA’s Cassini orbiter, which orbited the ringed planet from 2004 to 2017. While studying the planet, Cassini also flew past Enceladus nearly two dozen times, taking images of the moon and analysing it with its instruments. Its observations revealed an icy world the size of the United Kingdom, a giant snowball floating in space, covered in cracks and fissures known as tiger stripes.
But the spacecraft’s crucial discovery was that there appeared to be jets of water regularly firing into space from the moon’s south pole. Scientists suspect this is water being ejected from a subsurface ocean trapped beneath the icy shell of the moon. A source of heat, such as the tidal push and pull of Saturn, appears to be melting ice inside the moon, creating a giant ocean that might encompass the entirety of Enceladus.
This ocean is thought to begin about 10 kilometres (six miles) beneath Enceladus and extends down to the moon’s rocky core. This is vital because the core might be generating heat at the base of the ocean. That, in turn, might power hydrothermal vents, openings where hot water rich in minerals flows outwards, possibly supplying food for life to exist. On Earth, life is found near hydrothermal vents, and the same could be true on Enceladus.
In deep water
Accessing this ocean is difficult because of the thick ice above, but Cassini flew through plumes firing out from Enceladus at 1,300 kilometres (800 miles) per hour and studied some of that material coming out of the ocean. It found water vapour, carbon dioxide, and organic materials such as phosphorous (some of the building blocks of life) – but the spacecraft was not designed to study the plumes to look for direct signs of life.
That could come on a later mission, though. Scientists hope to one day return to Enceladus and study this ocean in greater detail. Some of the plume material falls back onto the surface of Enceladus, meaning that a lander could directly sample the ocean without needing to burrow down beneath the ice. Perhaps such a mission could discover direct evidence of life. If it did, it might suggest that frozen ocean worlds like Enceladus are a good location to find life. There are thought to be several moons with oceans trapped beneath them in our solar system, and they might be common around other stars, too. Perhaps the most abundant habitats for life in the universe are not stargazing creatures on the surface of rocky worlds like Earth but fish swimming in alien oceans.
If you prefer your habitable planets rocky instead of icy, you might want to turn your attention to Titan, Saturn’s largest moon, nearly half the size of Earth and larger than the planet Mercury. While Titan was first discovered in 1655, it wasn’t until some 300 years later that we discerned its most important characteristic: the Saturnian moon is coated in a thick atmosphere, the only moon in the solar system with such a feature.
That atmosphere makes Titan one of the most Earth-like places ever discovered. Composed mostly of nitrogen and methane, the atmosphere is thicker than the one that blankets Earth, giving Titan an atmospheric pressure 60% greater than our planet – so much so that, if you stood on its surface with wings strapped to your arms and flapped them, you could fly in its skies. But the atmosphere also enables Titan to have liquids on its surface, making it the only known world anywhere in the universe, apart from Earth, with lakes and seas.
Titan’s bodies of liquid, however, are nothing like Earth’s watery oceans. Instead, they are made of liquid ethane and methane, more resembling petroleum than water. So viscous are these liquids that, in some of Titan’s lakes and seas, waves only one centimetre (0.4 inches) in height protrude over the surface. The largest body of liquid on Titan, Kraken Mare, is about 500,000 square kilometres (190,000 square miles) in size – roughly as large as France.
The existence of these bodies of liquid was studied by the Cassini spacecraft’s radar instrument, which was able to bounce radar waves below the atmosphere to actually ‘see’ the liquids, and telescopes with infrared capabilities like the Hubble Space Telescope, which can peer through the haze. Cassini also carried with it a lander built by the ESA – called Huygens – which descended through the atmosphere of Titan and parachuted to the surface, the only landing ever conducted in the outer solar system. The lander’s camera revealed pebble-like rocks potentially weathered by liquids on the surface – but it didn’t directly observe a lake or sea.
While Titan’s surface temperature is about -179°C (-290°F), the moon also appears to have a weather cycle where methane rains from the skies before it evaporates. These rainstorms on Titan are likely rare: occurring less than once every 30 years, but are similar to the monsoon-like downpours on Earth.
Old school cool
Titan’s strange surface means it might closely resemble a primordial Earth, perhaps bearing similarities to what our planet looked like billions of years ago – indicating it has the prebiotic chemistry and the ingredients that allow life on Earth to arise. That might mean that simple microbial life has also been able to arise on Titan. Why the moon has remained in this primordial state, caked in smog, is unclear but might be due to its distance from the Sun
Yet the moon might also have a subsurface ocean hidden beneath its surface, similar to Enceladus. This means there are not just prospects for life hidden on Titan’s surface but also under its surface. This possibility led NASA to begin a mission called Dragonfly, which is scheduled to launch towards Titan in 2028 and arrive in 2034. Dragonfly is a unique space vehicle, a quadcopter – or drone – that will fly in the skies of Titan, enabling scientists to study different locations on the surface in the hunt for evidence of past or present life.
Dragonfly will build on the success of a similar but much smaller drone, Ingenuity, which landed with the Perseverance rover and flew in the skies of Mars 72 times from 2021 to 2024. It will land near a crater on Titan called Selk, where liquids might have mixed with organic material, allowing primitive life to arise. Exactly what Dragonfly will see is unclear because much of Titan’s surface remains obscured by its atmosphere. Perhaps hidden rivers and lakes await discovery – and with them, tantalising prospects of life.
Both Titan and Enceladus, in their own ways, are exciting and potentially habitable locations. As we continue to find planets around other stars, perhaps these two worlds might give us a good indication as to which might be capable of hosting life. In our own solar system, they might be two of the best places to look for any companions we didn’t know we had.

Enceladus

Year of discovery: 1789
Diameter: 500 kilometres (310 miles)
Distance from Saturn: 238,000 kilometres (148,000 miles)
Surface temperature: -201 Celsius (-330 Fahrenheit)
Length of day: 1.4 Earth days
Key features: Icy crust, subsurface ocean
Spacecraft visited by: Pioneer 11, Voyager 1, Voyager 2, Cassini-Huygens

Enceladus jets out
water vapour
from its southern pole.

“The lander’s
camera revealed
pebble-like rocks
potentially
weathered by
liquids”

Titan experiences
raging dust storms
at its equator.

Titan

Year of discovery: 1655
Diameter: 5,150 kilometres  (3,200 miles)
Distance from Saturn: 1.2 million kilometres (759,000 miles)
Surface temperature: -179 Celsius (-290 Fahrenheit)
Length of day: 15.9 Earth days
Key features: Thick atmosphere, lakes and seas
Spacecraft visited by: Pioneer 11, Voyager 1, Voyager 2, Cassini-Huygens

Titan has liquid
oceans of hydrocarbons
like ethane on
its surface.

Discover the mysterious Oort cloud, a distant
realm of enigmatic icy bodies at the very
edge of our solar system, shaping comets
and puzzling some of our greatest thinkers

Far beyond the planets we know about in our solar system lies a mysterious and distant frontier. Its name? The Oort cloud. It may sound like something from an old episode of Star Trek, but it’s very real.
This faraway pod of icy bodies surrounds our solar system, marking the boundary between our Sun’s gravitational influence and interstellar space. Though no human has ever observed it directly, the Oort cloud (named after its discoverer, the Dutch astronomer Jan Oort) is thought to play a crucial role in shaping long-period comets and preserving ancient remnants from the solar system’s formation.
It represents one of the last great mysteries of our cosmic neck of the woods, capturing the imagination of professional scientists and amateur stargazers alike. So what exactly is the Oort cloud? And why, most importantly, is it so astronomically significant?
The Oort cloud is a region consisting of trillions of icy objects known as ‘planetesimals’. Experts believe that these things form a spherical shell that encases the entirety of the solar system. This enclosure is so far away, however, that sunlight is incredibly faint there. Making objects in the Oort cloud practically invisible to us with our current array of telescopes. Even the most advanced technology, like the James Webb Space Telescope, is almost entirely incapable of detecting individual Oort cloud objects due to their distance, size and dimness.
Outer wilds
The Oort cloud is believed to start around 1,000 astronomical units (AU) from the Sun and extend to as far as 100,000 AU away. To put this into perspective, one AU is the average distance between Earth and the Sun, about 93 million miles (150 million kilometres). The Oort cloud’s outer edge reaches almost halfway to Proxima Centauri, our nearest stellar neighbour.
What makes the Oort cloud particularly intriguing is its composition. It consists mainly of frozen water, ammonia, methane and other volatile materials. These icy bodies are the building blocks of long-period comets, which can take hundreds, thousands or even millions of years to orbit the Sun. Unlike the Kuiper belt - a disc-shaped region of icy objects beyond Neptune - the Oort cloud’s structure is thought to be globular, reflecting the gravitational scattering processes that most likely shaped it.
Despite being unobservable today, the Oort cloud’s existence is strongly inferred from the trajectories of long-period comets. These comets often appear to come from a distant, isotropic region which surrounds the Sun, matching the theoretical predictions for the Oort cloud. Understanding this enigmatic region could, it is believed, help us piece together the dynamics of cometary paths.
So, how exactly did the Oort cloud come about? Well, it seems likely that the structure formed during the very, very early days of the solar system. We’re talking approximately 4.6 billion years ago. As the Sun coalesced from a dense molecular cloud, leftover material began to form planets, moons, asteroids and comets. Some of these icy bodies were scattered outward by gravitational interactions with the forming giant planets, such as Jupiter and Saturn.
As these objects were flung into the far reaches of space, the Sun’s gravity kept many of them loosely bound, creating the vast, distant reservoir we now call the Oort cloud. It’s also very possible that nearby stars and interstellar forces contributed to its formation, too – pulling or capturing material into the solar system’s outermost regions. This dynamic and chaotic process has left the Oort cloud as a kind of cosmic time capsule, preserving material from the solar system’s earliest days.
Thing of the past
Given that it’s stamped with a kind of theoretical status, it might be tempting to dismiss it as not all that important. However, the Oort cloud holds quite significant scientific interest for several reasons. It provides a unique glimpse into the history of our solar system. The icy bodies within it are thought to be some of the most pristine remnants of the early solar nebula, offering clues about the conditions that existed during the solar system’s formation. Gifting clues into the broad history of our solar system’s formation, understandably, excites the astronomical community in no small measure.
The Oort cloud is more than just a distant shell of icy objects, though: it could also be key to unlocking some of the most profound questions about our solar system’s future.
As the Sun ages, interactions between The Oort cloud and passing stars or other galactic forces could redistribute its objects, potentially delivering water or organic material to other planetary systems (and maybe even our own). Understanding these processes helps predict the solar system’s long-term sustainability.
Not only that, but the Oort cloud’s role in protecting inner planets from cometary impacts - all while being a valuable source of life-sustaining molecules - could offer up clues as to its impact on Earth-like planets’ potential to host life.
Cloud Nine
One of the Oort cloud’s most visible contributions to our solar system is its role in producing long-period comets. These celestial travellers, such as Comet Hale-Bopp, bring with them a wealth of scientific data. As comets approach the Sun, their icy surfaces sublimate, releasing gases and dust that form their characteristic tails. By analysing these materials, scientists can study the building blocks of planets and investigate whether comets might have played a role in delivering water and organic compounds to Earth, potentially jump-starting life on our planet.
The Oort cloud is also significant from a planetary defence perspective. While the risk of a comet impacting Earth is relatively low, the potential consequences could be unfathomably catastrophic, as evidenced by past mass-extinction events (you could ask the dinosaurs about that if they hadn’t all been wiped out). Understanding the Oort cloud’s dynamics and the factors that send comets hurtling toward the inner solar system is crucial for assessing and mitigating these kinds of risks.
Beyond its scientific importance, the Oort cloud holds a symbolic significance as the outermost boundary of our Sun’s influence, too. It marks the edge of the heliosphere and serves as a natural transition to interstellar space, almost like a checkpoint. By studying this boundary, we can better understand how our solar system interacts with the galaxy at large, shedding light on the processes that shape stellar ‘neighbourhoods’.
One way of looking at it is that the Oort cloud is like a kind of cosmic library, preserving the stories of our solar system’s origins while posing tantalising questions about its role in our planet’s development and future. Its study represents a frontier that challenges our brightest minds’ ingenuity and curiosity, pushing the limits of what we know and inspiring future explorations into the farthest reaches of space.
Because direct observation is currently impossible, scientists are forced to rely on more indirect methods of monitoring the Oort cloud, such as studying the trajectories of long-period comets. This remains our primary tool for understanding this enigmatic icy region. Future space missions equipped with highly sensitive instruments might one day provide more concrete evidence of the Oort cloud.
Currently, the Oort cloud stands as a symbol of the unknown, challenging our understanding of the solar system and its boundaries. Its influence is evident in the comets that grace our skies and the questions it raises about our place in the cosmos.
As technology advances and our reach extends further into space, the Oort cloud may one day begin to reveal its secrets, offering unparalleled insights into the history of our solar system and the dynamic forces that shaped it. Until then, this mysterious field remains a curious glacial frontier, beckoning us to explore further and reveal things we all really Oort to know about.

We can infer the
existence of the Oort
cloud thanks to its
influence on comets.

The Oort cloud is distant –
residing beyond the
Heliopause and well on
the way to Proxima Centauri.

© Adobe Stock (AI Generator); Getty; NASA / JPL-Caltech

Journeying to

The Oort cloud

We will have sent an object into the Oort cloud… in 300 years time

Launched back in 1977, NASA’s Voyager 1 and Voyager 2 spacecraft set off from NASA’s Kennedy Space Center at Cape Canaveral in Florida with a mission to traverse beyond the outer solar system, venturing into interstellar space. Voyager 1 entered this region on 25 August 2012, followed by Voyager 2 just over six years later.
Travelling at approximately 3.6 AU per year, Voyager 1 is currently about 162.8 AU from Earth. Despite their vast distances from Earth, the Voyagers are still yet to reach one of their major destinations, the Oort cloud.
It’s projected that the Voyagers will take roughly 300 years to reach the inner boundary of the Oort cloud and potentially - get this - about 30,000 years to traverse beyond it. So don’t expect news of a Voyager report on the Oort cloud coming our way any time soon. Certainly not in our lifetimes, anyway.

It’s thought the Oort cloud
holds material that can
help us understand the
solar system’s formation.

Long-period comets
originate from the Oort
cloud while others come
from the Kuiper belt.

Exploded solar system
diagram showing the
asteroid belt, Kuiper belt
and the Oort cloud.

“Currently,
the Oort cloud
stands as a
symbol of the
unknown”

v

Will humans set foot on another planet?

The crew for Artemis II,
who will orbit the
Moon in 2026.

We’ve been to the
Moon, and plan to
return this decade.
But what prospects do
we have for exploring
other worlds?

In the history of humanity, our species has only ever set foot on the surface of two worlds in the universe – Earth and our Moon. In a galaxy filled with hundreds of billions of stars and likely trillions of planets, this doesn’t seem too impressive. What hope do we have of exploring the cosmos if we can barely take a baby step in our own solar system?
Yet, do not let humanity’s tentative steps into the unknown get you down. Today, we are advancing our pedigree in space like never before, advancing our modes and methods of space travel to greater and greater levels. In your lifetime, you probably won’t see humans walk on the surface of, say, an alien Earth around a star thousands of light years away. But perhaps you will see the first steps on a world closer to home, Mars, and with it, the promise of grander adventures to come.
Full blast
The Apollo missions of 1969 to 1972 showed that we can travel to other worlds if we set our minds to it. Now, we are hoping to return to the lunar surface. This decade, NASA’s Artemis program intends to once again land on the Moon, this time with permanent settlements in mind. Working with private companies like SpaceX and Blue Origin, the goal is to create a lunar economy that supports long-term habitation of the Moon’s surface. This is being undertaken with a grander goal in mind: landing the first humans on Mars. We know the Moon has vast quantities of water ice at its poles, trapped in permanently shadowed craters that never see sunlight. If we can land on the Moon and mine this resource, we might be able to split apart the hydrogen and oxygen in the ice to make rocket fuel, which could then be used to propel new spacecraft out into the solar system.
Seeing red
Mars is our best bet for standing on another planet for now. Let’s consider Mercury, for comparison. Orbiting much closer to the Sun – 57 million kilometres (36 million miles) away, nearly three times closer than Earth and six times closer than Mars – it is battered by the Sun’s radiation. With no atmosphere, its surface goes through extreme temperature variations from -180°C (-290°F) at night to 430°C (800°F) in the day. The planet’s proximity to the Sun also means that it experiences the equivalent of the worst solar storm recorded on Earth every single day. This combination of deadly solar radiation and extreme temperatures makes the prospects of humans ever setting foot on Mercury, or even really wanting to, a remote possibility.
It is also hard to get to Mercury; being so close to the Sun, a spacecraft must find a way to reduce its speed over many years, using flybys of Earth, Venus and Mercury itself to slow the speed of the spacecraft via the gravitational pull of each planet to enter orbit. The plan for Europe and Japan’s uncrewed BepiColombo mission to Mercury, for example, launched in 2018, required nine flybys of all three planets to enter its upcoming orbit eight years later in 2026.
Further out from the Sun, we find the equally inhospitable Venus. Here, the planet’s thick atmosphere makes the world a veritable hellhole. Some 93 times denser than Earth’s atmosphere, the pressure on the surface of Venus from this broad expanse of cloud is nearly 100 times more crushing than Earth’s surface. Temperatures on the planet’s surface, meanwhile, are 465°C (870°F) on average, hot enough to melt lead. This deadly combination makes life on the surface of Venus all but impossible.
In the skies of Venus, however, some 50 to 65 kilometres (30 to 40 miles) above the surface, conditions are more moderate. Here, the temperature and pressure more closely resemble that on Earth, leading some scientists to suggest that airborne microbial life could exist in this region, hinted at by the detection of phosphine on Venus – a gas produced by some simple forms of life – in 2020. Here, it’s theorised that humans in floating habitats lofted by balloons could also reside relatively comfortably. We might not want to set foot on Venus any time soon, but maybe floating in its skies isn’t the worst option.
Shoot for the moons
The other planets of our solar system – Jupiter, Saturn, Uranus, and Neptune – are gas giants with no solid surface humans could stand on. Some of their moons might be more promising for human exploration, however. Jupiter’s icy moon Europa, for example, is thought to have a subsurface ocean that humans might be eager to visit – although its surface is bathed in intense radiation from Jupiter. Saturn’s frigid moon, Enceladus, is similarly alluring and dangerous, for the same reasons. But its largest moon, Titan, is more promising, with a slightly thicker atmosphere than Earth’s. It is the only known world in the universe aside from Earth with bodies of liquid on its surface, although here, they are more similar to lakes and seas of petroleum than water. Nonetheless, the moon is relatively habitable when compared to, say, Mercury and Venus. Perhaps far in the future, it might be a world humans set foot on.
In the near future, Mars is our most promising target. Half the size of Earth and with a thin atmosphere of mostly carbon dioxide, the planet today is a relatively barren wasteland, but one that humans in a spacesuit could step foot comfortably on. The planet’s gravity is about a third that of Earth, with an average surface temperature of about -65°C (-85°F). While its pressure is much lower than Earth’s, with adequate equipment, there is no reason a human could not survive on Mars.
Guiding star
Companies like SpaceX in the US are hoping to make this a reality. Its new rocket, Starship, is designed with human habitation on Mars in mind. The structure is in two halves, an upper part that could house humans and a lower giant booster, and both are reusable. SpaceX is under contract with NASA to use Starship to return humans to the Moon this decade. But the idea is that the rocket could also be used for landings on Mars, perhaps using some of the methane on the planet as rocket fuel too.
Getting humans to Mars on Starship will be challenging. It will require significant financial investment from the US, and considerable political will to make such a mission happen. The technical challenges are also large. We have landed rovers and stationary landers on Mars before, but never something as large as Starship. How will we ensure the rocket passes through the atmosphere and safely touches down on the surface? Can we land something that big on Mars? We have also never lifted off from Mars, a necessary requirement if we want humans to be able to return home to Earth. Starship is designed with this goal in mind, but it will still be an enormous challenge. How long will it take to build launch and landing pads to support Starship on Mars? And who will want to go to Mars on those first missions and test those systems for the first time?
If successful, however, SpaceX wants humans to permanently settle on Mars, using Starship as their means to travel to the planet and return to Earth. If this idea comes to fruition, perhaps one day in decades to come people really will be travelling between Mars and Earth, with futuristic habitats and eventually cities supporting those brave settlers who choose to make that journey to the Red Planet. It will be no small feat to make this happen, however.
Further afield, what prospects do we have for setting foot on worlds around other stars? This has long been the promise of science fiction – anyone who has watched Star Trek has imagined humans one day setting foot on alien planets for real. But given the vast travel times in the universe, can we actually make this happen? With our current rocket technology, getting to Mars is a reasonably long journey of about eight months. Reaching Jupiter takes some five years, Saturn about seven years, while Uranus and Neptune take about 16 and 30 years to enter orbit respectively. If you want to get to another solar system those journey times get considerably more daunting.
Our closest star, Proxima Centauri, is 4.2 light years from Earth. Neptune, for comparison, is four light hours away. With rockets today, like Starship, a journey to Proxima Centauri would take tens of thousands of years to reach. The only hope for humans to survive such a mission would, either, to be placed in deep hibernation – something we aren’t sure is possible – or to live on a generational spaceship, living through multiple generations of humans that eventually reach the star.
Instead, we might need to pin our hopes on futuristic forms of space travel that could reduce the journey time. One idea that has been touted is to use vast solar sails to accelerate a spacecraft to extreme speeds. Using such a sail, a spacecraft could be slowly but constantly accelerated by our sun, potentially reaching speeds of up to 130 million miles (210 million kilometres) per hour, or about 20% the speed of light. At such speeds, a journey to Proxima Centauri might take decades, not millennia.
Another possibility would be more hypothetical forms of travel, such as the warp drives employed in Star Trek. While science fiction for now, some scientists have suggested it could be possible to develop engines far in the future that could bend space-time. These could theoretically produce a warp bubble around a spaceship that compresses space in front of it and expands it behind, enabling extreme velocities to be achieved perhaps even faster than the speed of light. For now, such ideas are entirely theoretical. Beyond the solar system
If we can solve the problem of long-distance space travel, there is no reason to think we could not walk and even live on other planets. Already today we know of about 6,000 planets around other stars, exoplanets, many of which are rocky worlds. Some of these also reside in a star’s habitable zone, the region of a star where temperatures are just right for liquid water to exist. If we could find a way to reach these planets, perhaps they would have habitable surfaces like our own complete with vegetation and maybe even life.
And if we can reach one exoplanet, why not many? Humans could then find themselves not just confined to one solar system but visitors to many. Maybe the question then wouldn’t be will humans set foot on another planet, but which planet will we not set foot on? Perhaps some, with advanced or intelligent life, would be off limits so as not to cause any disruption. Perhaps others would become worlds of scientific study. Others still, if a species were advanced enough, could be locations where we make contact with other beings, setting foot on planets together with alien civilisations.
Those are possibilities for the long term. For now, we have the small matter of returning to the Moon first, and then possibly to Mars, on the horizon. If all goes to plan, exploration of those two worlds will take place in the coming years and decades. Our first steps on another planet, while barren and inhospitable today, will be the start of a journey that might one day see humans inhabit the stars.

It’s hoped that humans
will establish a
permanent base on the
lunar surface.

Artemis

This decade, NASA is planning to return humans to the Moon as part of its Artemis program. Beginning with a flight of four humans around the Moon, the program will then see humans set foot on the lunar surface once again. The goal is to then develop permanent settlements that humans could inhabit, similar to how we have lived in space on the International Space Station (ISS) for two decades.
The target for this lunar exploration is the south pole of the Moon, where abundant water is thought to be trapped as ice in craters. If humans can land and extract this ice, they might be able to turn it into rocket fuel, enabling journeys to other destinations such as Mars and possibly beyond. While a grand goal for now, Artemis is hoped to be the pathway that will set us in this direction and make humans a true multi-planetary species.

The launch of Artemis I,
an unmanned flight
using the Space Launch
System (SLS).

XXXXXXXXXXXX

“We know
the Moon has
vast quantities
of water ice at
its poles”

SpaceX’s Starship will
take humans back to the
Moon, and potentially
further to Mars.

Amazon’s Jeff Bezos
demonstrated a future
of space tourism with
his Blue Origin craft.

A test flight of Starship
taking place on 6 June
2024 in Brownsville, Texas.

“SpaceX wants
humans to
permanently
settle on Mars,
using Starship as
their means of
travel”

The most realistic
future goal for human
spaceflight is to land
humans on Mars.

Ideas of an orbital hotel
have been floated.
We expect all rooms
will have a sea view.

Space

tourism

Today, humans are undergoing space travel frequently, not just on government-led missions, which have taken place for decades, but on commercial missions. Companies like SpaceX and Blue Origin are enabling wealthy individuals to pay for journeys into space, with the hope that this will lead to more affordable space travel for others on Earth.
Currently, paying customers can either pay for short hops into space lasting minutes or longer missions with orbits lasting days or more. In the near future, it might then be possible to also pay to travel to the Moon, perhaps visiting one of the permanent settlements NASA and other agencies are hoping to set up there. Further afield, people might be able to pay to travel to other planets, such as Mars, and become temporary visitors or even settlers, giving up their lives on Earth to become permanent inhabitants of another world.

Explore beyond our solar system
into the wider universe

62

78

62 how are galaxies formed?
68 black holes
74 quasars
78 The great attractor theory
82 When Galaxies Collide

The process of galaxy formation is still
something of a mystery, but astronomers have
pieced together a rough picture of how it must
have happened

Written by Andrew May

If you consider the entire observable universe, then galaxies – around two trillion of them – can be thought of as its basic building blocks. But each galaxy is almost a mini-universe in its own right, containing billions or trillions of stars, many of them with planetary systems like our own solar system. As well as stars, which are their most obvious constituent when observed through an optical telescope, galaxies also contain more diffuse matter in the form of clouds of gas and dust.
Some galaxies, particularly smaller ones, can be quite irregular in appearance, but the majority have neat-looking symmetrical shapes that fall into two broad categories, called ellipticals and spirals. Visually, the latter category is particularly impressive and probably what most people think of when they hear the word galaxy. These are basically thin circular discs with prominent spiral arms containing large quantities of gas out of which bright new stars continuously form. In contrast, elliptical galaxies have three-dimensional shapes, ranging from rugby balls to slightly flattened soccer balls, and contain much less gas and dust than spirals. As a result, there’s far less star formation – and hence, a much higher proportion of old stars – in ellipticals.
Our galaxy is a fairly typical spiral galaxy in which the Sun is just one of around a hundred billion stars. Most of these lie in a thin disc around 100,000 light years across, with our solar system located about 26,000 light years from the galactic centre. The density of stars increases dramatically towards the centre, where there’s a bulge in the disc somewhat like a miniature elliptical galaxy. Ironically, from our viewpoint inside it, we can’t see our own galaxy as clearly as many external galaxies. What we do see is a faint streak of light stretching across the night sky, which the ancients dubbed the Milky Way – a name that’s now applied to the galaxy as a whole.
Since galaxies are such a fundamental component of the universe, it’s reasonable to ask where they came from and why they take on different shapes. But this turns out to be a surprisingly difficult question to answer. In the case of stars within the galaxy, we see them at all stages of their life cycle, from birth to death, with new stars being formed all the time. So astronomers have a pretty good understanding of the processes involved in the formation and evolution of stars. In the case of galaxies, however, the same isn’t true. For the most part, they were formed very early in the universe’s vast history, and although galaxies do evolve over time, they don’t go through a repeating cycle of birth and death in the same way that stars do. So, working out how galaxies formed has to be based on a mixture of theory, computer simulation and maybe even a little guesswork.
Come together
Evidence from the cosmic microwave background – radiation that was emitted less than half a million years after the Big Bang – shows that at that time, around 13.7 billion years ago, the universe was almost completely uniform in terms of the distribution of material within it. By that stage, the universe was already in a state of rapid expansion, and as it grew in size, the particles of matter – mostly hydrogen atoms – became more and more thinly spread out. But there was another effect competing with this expansion: gravity. Essentially, a force that tends to draw different pieces of matter together, this had the effect of pulling some of the diffusely expanding material into denser clumps.
In some cases, these clumps were substantial enough that their gravity held them together against the general expansion. In turn, their gravity was able to pull in further material, allowing them to grow. As the mass of a clump increased, so did its gravitational pull, thus causing it to collapse down to smaller dimensions and higher density. This resulted in the first ‘proto-galaxies’, which appeared within the first million years of the universe’s existence – a blink of an eye compared to its present age of over 13 billion years.
The first comprehensive theory of galaxy formation, dating from 1962, was based on the assumption that the principal component of proto-galaxies was hydrogen gas. While that was entirely reasonable, at the time, it’s come to look rather too simplistic in terms of what we know today. As a result, that early theory – now referred to as the ‘top-down’ model – has largely been superseded. Even so, many of its basic elements still hold today, so it’s worth taking a quick look at it.
In the top-down model, the first proto-galactic clumps to collapse took the form of giant gas clouds, each of which was comparable in overall mass to a present-day galaxy. As this cloud of gas collapsed under its own gravity, its density increased, allowing some early star formation to occur. So the first generation of stars actually formed before the galaxy stabilised, at a time when other parts of the gas cloud were still collapsing. After this point, the gas and stars behaved in subtly different ways. To put it in simple terms, molecules in a collapsing gas cloud can collide with each other and lose energy in the process, while stars almost never collide with each other – and hence, they retain the same energy they started with.
Seeing stars
The consequence, as far as the gas is concerned, is that it collapses ever inward towards the central parts of the galaxy. But while it can lose energy, it can’t lose its rotational motion, ultimately settling into a flat, rapidly rotating disc. After this happens, new stars can form within this disc, and these are the bright young stars we see in spiral galaxies. As for the older stars that formed while the proto-galaxy was still collapsing, these retain most of their original motion, and so, rather than ending up in a thin disc, they create a three-dimensional structure. This might be the central bulge of a spiral galaxy or – if most of the proto-galactic gas was used up in an early burst of star formation – an entire elliptical galaxy.
As a broad-brush picture of galaxy formation, that early 1960s theory is still valid today. But at least one important discovery has been made since it was formulated meaning the top-down model can’t provide a complete picture. The main problem is the role played by the mysterious substance known as ‘dark matter’. This has never been seen through a telescope, however, astronomers believe it ought to exist because it’s the most straightforward way to explain certain peculiarities in the observed motion of visible matter in galaxies. The idea is that dark matter is an invisible substance that permeates a whole galaxy and interacts with its visible components solely through its gravitational attraction. And it’s not a minor ‘correction factor’, either. In order to explain all the observed data, dark matter has to make up as much as 80% or more of a galaxy’s total mass. NASA describes dark matter as the ‘scaffolding of the universe’ – with visible matter, in the form of stars, gas and dust, simply collecting inside this scaffolding.
If this is the case, then it’s clear that the existence of dark matter calls for a rethink of the way galaxies form. The result is an alternative (and now widely accepted) theory called – perhaps unsurprisingly – the ‘bottom-up’ model. Unlike the top-down model, this assumes that the gravitational force that pulled the first proto-galaxies together came from dense clumps of dark matter rather than the self-gravity of the hydrogen gas destined to form the visible portion of the galaxy. In this view, the initial progenitors of galaxies were much less massive than present-day galaxies, and it was only over the course of time that they gradually grew to their current size through the accretion of further material, together with repeated mergers of smaller galaxies into larger ones.
Centre of attention
Another recent development that may impact our picture of galaxy formation is the discovery that the centres of most galaxies harbour supermassive black holes. They’re called ‘massive’ because of the large amount of matter they contain, not because of their physical extent – which is actually very small. While the largest of these black holes are billions of times as massive as a single star, they occupy a region of space that’s tiny when compared to a star cluster containing the same number of stars. As a result, these dense concentrations of gravitating matter dominate their surroundings, and it’s possible they may predate the galaxy around them. In this view, the central part of a proto-galactic gas cloud collapsed all the way down to a black hole, and the rest of the galaxy subsequently accumulated around it. Alternatively, it’s possible that the galaxy formed first, with the first generation of stars ending their lives as relatively small black holes, which then merged with each other to produce a single supermassive one.
Although the era of galaxy formation occurred in the distant past, that doesn’t mean it’s impossible to observe galaxies currently undergoing formation, or at least very early in their lives. This is because light travels at a finite speed, so when we observe a distant object, we see it as it was some time in the past. A galaxy that’s ten billion light years away, for example, appears to us the way it was ten billion years ago. According to the bottom-up model, this means that very distant galaxies should look distinctly smaller and less well-structured than nearby ones. To a large extent, this expectation is borne out by observations made by NASA’s Hubble telescope, and, more recently, by its successor, the James Webb Space Telescope (JWST). However, recent observations by the latter have turned up discrepancies that seem to indicate that the bottom-up model isn’t the last word on galaxy formation.
Follow the evidence
The problem is that when the theory is worked out in detail, it doesn’t just say that most newly-formed galaxies should be small and scrappy-looking – it says that all of them should be. And that’s not what JWST sees. At least some of the earliest galaxies it has observed are as big and bright as present-day ones. This suggests that, in order to explain the observed evidence, astronomers are going to have to modify their thinking surrounding galaxy formation. This may be as simple as tweaking the bottom-up model to fit the observations, but it might end up being more revolutionary than that.
The original motivation for the bottom-up model was to reconcile galaxy formation with the existence of dark matter – which, in turn, was postulated in order to explain certain otherwise anomalous astronomical observations. While dark matter is largely accepted within the scientific community, a minority faction has pushed an alternative explanation of anomalous observations – a different theory of gravitation known as ‘modified Newtonian dynamics’. While this theory has weaknesses of its own, it does have the advantage of predicting that large galaxies – formed in accordance with the old top-down model – should exist in the very early universe. And that’s just what JWST appears to have observed.

© Adobe Stock (AI Generator)

Spiral galaxies are not
only visually distinct,
but they also continuously
birth new stars.

“around
13.7 billion years
ago, the universe
was almost
uniform”

A depiction of galaxy
formations using the
top-down model
(above) and
bottom-up (below).

Clusters and

Superclusters

In terms of its large-scale structure, the universe can be pictured as a hierarchy in which galaxies form the bottom rung. Next up the chain are groups or clusters of galaxies, typically made up of a few hundred individual galaxies that are held together by their mutual gravity. These are the largest structures that are gravitationally bound – the next level up, superclusters, are more tenuous groupings of galaxy clusters that aren’t physically bound to each other. In turn, these superclusters are grouped into ‘walls’ – huge sheet-like structures that are typically ten to 20 million light years thick and up to a billion light years in extent. But even these giant walls aren’t the biggest structures in the universe. They’re arranged in a vast ‘cosmic web’, made up of twisting filaments with almost empty voids in between and denser structures like superclusters occurring at the intersections of the filaments.

© NASA / ESA; Adobe Stock; Getty

Cosmic Time Machines

All telescopes can be thought of as time machines because they show us astronomical objects as they were at some point in the past. The more powerful a telescope is, the further it can see into the depths of the universe – and hence further into the past. The most distant galaxies are extremely faint, so they require very long exposure photographs with a minimum of background light pollution. That’s why astronomers prefer to use space telescopes like Hubble or JWST rather than ground-based ones. Another trick they can use is ‘gravitational lensing’, in which intervening matter between a distant galaxy and ourselves focuses its light to produce a much brighter image than we would otherwise see. The JWST team used this effect in 2024 to view a distant galaxy called the Firefly Sparkle, which is believed to look very much like our own galaxy did in its younger days.

A Hubble image of
NGC 5264, a dwarf
galaxy with an
irregular shape.

How Are Galaxies Formed?

Galaxies play host
to trillions of stars,
with more being
formed all the time.

“it’s not
impossible to
observe galaxies
undergoing
formation”

Our own Milky Way
galaxy is spiral shaped
and is 100,000 light
years across.

© NASA; Adobe Stock; Getty

Of all the brain-bending things you
can think of, they will have nothing on
black holes. Prepare to have your mind
blown when discovering more about
these monumentally mystifying marvels
of outer space

The name ‘black hole’ is elegantly deceptive. As light can’t escape these gravitational voids, they are indeed, black – pitch black. But ‘hole’ suggests a whole lot of nothingness when, in fact, a huge amount of matter – gas, stars and dust – is squeezed into a relatively crammed space. It’s been posited that black holes are areas where gravity is strong enough to warp space, bend light and distort time, making them some of the most peculiar, intriguing and downright unfathomable objects in space. That’s saying something, seeing as how much other mind-blowing stuff is out there in our universe.
There are three main distinctions of black holes: the first we’ll discuss is stellar black holes. These are relatively small but extremely dense, meaning their gravitational pull is immense – any matter nearby doesn’t stand a chance. It’s been estimated that there are 10 million to a billion stellar black holes in the Milky Way, each ten to 24 times as big as our Sun. Some stellar black holes rotate near the speed of light, which warps space-time around them.
Intermediate black holes, meanwhile, are slightly different. It was once thought that black holes were either small or huge, but research suggests that there are mid-sized – or intermediate – black holes as well. They may be formed when stars in a cluster collide in a chain reaction. Unlike larger black holes, intermediate ones are elusive. Scientists search for them in dense star clusters, dwarf galaxies and even as potential rogue wanderers in space.
Then we have the celebrities you’ve likely heard of before: supermassive black holes. With a mass that’s millions or even billions times larger than our Sun, supermassive black holes are believed to be at the centre of every galaxy, including our Milky Way.
There’s also another ‘hypothetical’ type of black hole: primordial. Primordial black holes (PBHs) may have formed in the early universe, shortly after the Big Bang, making them among the oldest objects in the universe (if they exist, of course). They could have originated from extreme density fluctuations in the infant cosmos and range in size from tiny microscopic black holes to ones as massive as a star. Scientists are interested in PBHs because they might explain dark matter, mysterious cosmic radiation, and even the seeds of galaxy formation. Some scientists speculate that PBHs could be linked to unexplained cosmic events, like fast radio bursts or unusual gravitational wave signals. However, detecting them is extremely challenging due to their potential small size and elusive nature, so their existence remains unproven. If found, they could reshape our understanding of the early universe and fundamental physics.
The point of no return
So, how do we know black holes exist if they can’t be observed all that easily? Scientists have studied the matter surrounding them and discovered that one of the most prominent signs of a black hole is radiation from extremely hot gases just before they enter a black hole, and if any matter is unfortunate enough to pass close to a black hole, it will be ripped apart or devoured. It’s been theorised that if a person were to enter a black hole feet first, their body would be stretched like a thin piece of spaghetti as their feet would feel way more gravitational pull than their head. This difference in force – called tidal forces – stretches objects into long, thin shapes. This gravitational phenomenon is known as ‘spaghettification’, sometimes ‘the noodle effect’. But try not to think that things are ‘sucked’ into a black hole – that would suggest a black hole is a vacuum where it’s not. Instead, things fall into them, just as you would fall into a hole in a pavement.
Although black holes seem to be fascinating, if a little frightening, the way they are created is nothing short of awe-inspiring. After a star explodes in a supernova, it may collapse in on itself. As it shrinks, its gravity gets stronger, and a black hole is formed. Eventually, this gravitational pull is so strong that nothing within its reach can escape its clutches. The boundary between the interior and exterior of a black hole is called the event horizon. Once something crosses this threshold, it can’t leave. That’s it: game over. It’s a one-way ticket to oblivion.
Shedding light on dark stuff
The discovery of black holes wasn’t a single ‘Eureka!’ moment but rather a journey of ideas and observations over many, many years. It all started ramping up with Einstein’s theory of general relativity in 1915, which predicted that space could be warped so much by gravity that nothing – including light – could escape. But at the time, black holes seemed like just a wild theoretical concept rather than reality.
Fast-forward to the 1960s, when astronomers started finding evidence of incredibly dense objects in space, like Cygnus X-1, an X-ray source believed to be a black hole feeding on a companion star. Over time, more evidence piled up from gravitational waves and star movements, and finally, in 2019, the first-ever image of a black hole’s shadow was captured by the Event Horizon Telescope (EHT). In 2022, the image of the black hole at the centre of the Milky Way, called Sagittarius A*, was captured. The EHT Collaboration is also attempting to image the photon ring of a black hole – a region just outside the event horizon and beyond the accretion disk where light is bent into a circular orbit. Changes in a black hole’s appearance will also be observed, shedding light on phenomena such as jet formation and matter consumption.
So, what is the significance of these enigmatic, weird and wonderful holes that exist thousands, millions and billions of light years away from Earth? (To put it in perspective, Gaia BH1 is the closest known black hole from Earth, at about 1,560 light years away – and UHZ-1 is the most distant confirmed black hole, located more than 31 billion light years away.) The discovery of black holes has impacted our everyday lives in surprising ways. Studying black holes has pushed the limits of technology, leading to advancements in areas such as optics, data processing, medical imaging and GPS accuracy. Indeed, the same physics that helps us understand how black holes warp space and time also helps improve navigation systems that rely on precise time measurements. Black hole research has also inspired new materials and computing methods utilised in everything from security systems to smartphones. Notably, the complex data processing techniques used to study black holes – like combining signals from telescopes worldwide – have led to improvements in image-processing algorithms, which are now used in smartphone cameras to enhance photos and videos. And techniques developed to analyse black hole data have found applications the world of AI, helping with things like voice recognition and battery optimisation.
As well as helping you to get where you’re going without getting lost and boosting your photography, black holes are also pivotal in understanding how the universe works. These mysterious objects help to shape galaxies, influence star formation, and even warp space and time itself. Scientists study black holes to test the limits of physics and consider them to be ‘natural laboratories’ for testing theories of gravity and quantum mechanics under extreme conditions. In fact, researchers have recreated pseudo black hole models in labs to test all kinds of theories, like Stephen Hawking’s radiation theory (also known as Hawking radiation). Black holes might hold clues to some of the biggest cosmic mysteries, like dark matter and the universe’s origins. They are believed to be an essential element of how the universe continues to evolve and provide clues to its structure.
Puzzling Paradoxes
Thanks to technological advances, masses of research and the hard work of many scientists over the decades, there is a lot we now know about black holes. However, there’s still a whole bunch we don’t know. For starters, what actually happens inside a black hole? Once something crosses the event horizon – the point of no return – it’s gone forever, but we have no idea what happens next. Some theories suggest a singularity, where gravity becomes infinite, while others propose wild ideas like wormholes or entire new universes. Perhaps the events portrayed in episodes of Star Trek and the Christopher Nolan film Interstellar aren’t science fiction after all.
Then there’s the information paradox: does information that falls into a black hole get destroyed, or is it somehow preserved… somewhere? According to quantum mechanics, information can’t be lost, but black holes seem to break that rule. Stephen Hawking proposed that black holes emit radiation (the Hawking radiation we mentioned previously) and eventually evaporate. This raises questions about what happens to the information that falls into a black hole, challenging the principles of quantum mechanics.
We also don’t fully understand the role of black holes in cosmic evolution. How do they grow so fast or affect their surrounding galaxies and the structure of the universe over billions of years? When it comes to black holes, there’s so much left to uncover, and every new discovery throws up even more questions. They are full of mysteries that we haven’t yet – and may never – figure out. But that doesn’t mean we’ll stop trying. Discoveries are constantly being made about space and with recent revelations like the first-ever image of a black hole, we’re getting somewhat closer to unlocking their deepest secrets. Who knows? One day, they might even help us to understand what happens at the very edge of space and time.

Black Holes

© Getty; Adobe Stock

Radio image of
Cygnus A: the long thin
particle jets are produced
by the black hole at its centre.

“supermassive
black holes
are believed to
be at the centre
of every galaxy”

Concept of smaller
primordial black holes –
some of which could have
the same mass as Earth.

Hole

A

New world

The first-ever image of a black hole was revealed in April 2019 and was a groundbreaking milestone in science. Captured by the Event Horizon Telescope (EHT), it exposed the shadow of the supermassive black hole in the galaxy M87, about 55 million light years away. The image showed a glowing orange ring of light surrounding a dark centre – the event horizon – where gravity is so strong that not even light can escape.
This feat was monumental because black holes, by nature, aren’t visible. The breakthrough was an international effort, with more than 200 scientists using a plethora of observatories all over the world, from the South Pole to Hawaii, and cutting-edge techniques to piece together the data. Their efforts confirmed predictions from Einstein’s theory of general relativity and offered new insights into the physics of black holes and their role in the universe.

The galaxy Messier 106
has distinctive ‘arms’ that
could be made by its central
supermassive black hole.

© Adobe Stock; Getty; NASA; Event Horizon Telescope Collaboration

Galactic giants

Supermassive black holes are the largest type of black hole; the cosmic giants lurking at the heart of most galaxies, including our Milky Way. The one at the centre of the Milky Way is called Sagittarius A* (Sgr A*) and has a mass of over 4 million suns. Some of these monsters can even be billions of times bigger than our Sun! But despite their enormous mass, they’re incredibly compact. Scientists believe that supermassive black holes form over billions of years, possibly from the merging of hundreds or thousands of smaller black holes, or by devouring gas, stars and even other black holes. They play a huge role in shaping galaxies, influencing star formation, galaxy growth and holding galaxies together. Without them, the universe would look very different.

It’s understood
supermassive black holes
are capable of ejecting
bright jets of material.

Visualisation of two
black holes disrupting
gas at the centre of
a galaxy.

“Once something
crosses the
event horizon
it is gone
forever”

© Adobe Stock; Getty; ESO; NASA

Scientists are exploring distant planets,
analysing cosmic signals and studying extreme
environments on Earth to uncover evidence of
extraterrestrial life

There used to be a time when many astronomers would scoff at the mere idea of finding life elsewhere in the universe. But times have changed. Raising the questions of whether or not we are alone and whether or not we can locate life are now being more emphatically answered with a “no” and a “yes”. There is growing confidence that one day, there will be a massive news announcement that will stop Earthlings dead in their tracks.
Alien life has not yet been found. At least, we don’t think it has, scientifically speaking. There have, of course, been many claims of extraterrestrial beings visiting Earth – consider the infamous ‘spacecraft crash’ in Roswell, New Mexico, in 1947, and the many supposed UFO sightings, the likes of which date back as far as Egypt in 1450 BCE.
But those grainy shots of flying saucers don’t, in reality, cut it, and there have been numerous hoaxes that muddy the water. Those of a certain age will remember the film Alien Autopsy: Fact Or Fiction, which allegedly had footage of an extraterrestrial undergoing a post-mortem. That was revealed to have been filmed on an inexpensive set using plaster cast corpses filled with offal and raspberry jam.
Radio silence
In light of that, it’s still easy to laugh when the issue of life elsewhere is raised, but finding the existence of aliens is a serious business, with some top brains having dedicated much time to the task. In 1896, Nikola Tesla, a mechanical engineer, physicist and inventor, for instance, believed aliens could be transmitting signals. Three years later, he heard beeps on his apparatus, which caused some excitement, although more recent recreations have put that down to the natural intense kilometric emissions from Jupiter.
Even so, the idea had taken hold. In 1924, the United States held a National Radio Silence Day and asked citizens to turn off their radios for the first five minutes of every hour so that US naval stations could potentially better hear any Martian transmissions. Hopes were also high when NASA began looking for life beyond Earth shortly after it was established in 1958. By the mid-1960s, its scientists were searching for unnatural radio signals while watching for optical oddities and seeking strangely elevated heat signatures.
Backing up a lot of these efforts was a probabilistic argument put forward by Frank Drake, who worked at the National Radio Astronomy Observatory in Green Bank, West Virginia. Having developed a strong interest in the search for extraterrestrial intelligence (SETI), he used a radio telescope to monitor the stars Tau Ceti and Epsilon Eridani for alien signals six hours each day over four months. It became the first modern SETI search.
The following year, he formulated a crucial equation (the Drake Equation) to calculate the number of detectable extraterrestrial civilisations that may exist in the Milky Way galaxy – that is, those which may be transmitting a signal. It was made up of seven factors multiplied together: the number of stars created in the galaxy annually; the fraction of stars with planets; the number of planets per solar system capable of supporting life; the fraction of life-supporting planets where life appears; the fraction of life-bearing planets where intelligent life emerges; the fraction of planets with intelligent life that develops technology for communication; and the average length of time in years that those civilisations produce signs.
The search heats up
The equation has become hugely important in the search for alien life because it shapes the thinking about the likelihood of finding other civilisations in the universe. Indeed, Drake’s entire motivation was to stimulate scientific discussion, and after he unveiled the equation at the Green Bank Observatory to around a dozen eminent scientists and engineers, it arguably kickstarted the SETI movement. There was even some sort of secondary space race between the US and Russia, with both countries keen to pick up radio signals that indicated life elsewhere.
The search for alien life – intelligent or not – went further in the late 1960s when the Viking program was established. Finding evidence of life was one of the science objectives, so when a pair of probes eventually landed on Mars in 1976, they carried out biological experiments on the Martian soil. While they didn’t find anything definitive despite hopes that the planet had life-harbouring canals and rivers, they, at least, said NASA, found the elements essential to life on Earth: carbon, nitrogen, hydrogen, oxygen and phosphorus.
By then, NASA had also begun a more formal search for extraterrestrial intelligence, funding a study in 1971 that proposed building an array of as many as 2,500 radio telescope dishes so that radio signals could be detected at a distance of up to 1,000 light years. The plan – called Project Cyclops – was eventually cancelled, but it nevertheless affirmed the space agency’s intention. It also underlined the belief that alien life could exist.
Despite that, NASA began to hold back to some extent. Its SETI project was still small, and it wasn’t being handed huge amounts of money. As a consequence, Thomas Pierson and American astronomer Jill Tarter incorporated the SETI Institute, a not-for-profit research organisation, in 1984, intending to give searches the attention and funding they felt they required.
The first trustees included Drake and well-known astronomers such as Carl Sagan and the Nobel Prize winners Baruch Blumberg and Charles Townes. Since then, the SETI Institute has grown hugely – particularly after Congress cancelled funding for a high-resolution microwave survey of the sky in 1993, ending NASA’s SETI program. Its scientists use many instruments in their searches, including the Allen Telescope Array, WM Keck Observatory, the Very Large Telescope in Chile and the Hubble, Kepler and TESS space telescopes. Techniques have also changed hugely over the years, and where past searches have been disappointing, there is now fresh hope.
The Goldilocks Zone
A lot of attention focused on celestial bodies in the so-called Goldilocks zone – the area around a star where a planet’s surface temperature would be neither too hot nor too cold, thereby allowing liquid water to exist on the surface. The extent of that zone differs from one star system to another, and it depends on the size and heat of the host star. Of the bodies found in these zones, the smart money is on rocky planets of a similar size to Earth, being most likely to harbour life. But as scientists will attest, life doesn’t necessarily need to be confined to planets.
Mars remains a candidate for life, whether past or present. As such, the Curiosity Rover, part of NASA’s Mars Exploration Program, is searching for organic compounds and biologically derived molecules, while its successor, the Perseverance Rover, has been collecting samples from the dry bed of a potential former lake. In July 2024, NASA announced that Perseverance had detected a Martian rock with features and chemistry that could have been produced by ancient microbial life. Scientists also believe there may be liquid water hidden beneath Mars’s southern ice cap, which could prove very exciting in the search for alien life.
Even so, there’s just as much interest – perhaps more so – on a mission to Jupiter’s moon Europa. Like Saturn’s satellite Enceladus, Europa is thought to have a liquid water ocean beneath a thick icy crust, and it’s likely prevented from freezing by an internal heat source that could be released via hydrothermal vents, which are like hot springs. On Earth, despite a scorching 400°C heat, hydrothermal vents are home to bacteria, which use chemicals from the vent fluid to create sugar from carbon dioxide. This produces food, which has allowed some weird and wonderful species to thrive. These creatures are not intelligent, but they are examples of life, and similar forms may exist on the extraterrestrial moons.
NASA’s Europa Clipper launched in October 2024, and it’s due to reach Europa in April 2030, where it will perform a series of flybys while in orbit around Jupiter. There are also plans to go to Enceladus – one such mission, Enceladus Orbilander, is due to launch in the late 2030s, and the aim is for a probe to spend 18 months orbiting the moon and sampling its water plumes. Such missions will take time, however, so while those are ongoing or being planned, scientists are using other methods to find alien life and extraterrestrial intelligence.
One way of determining intelligent life on exoplanets is to look for technosignatures, which is what astronomers have been doing when seeking radio signals. These could be giveaway signs of life and include laser emissions, pollution and large structures. They are indicative of a civilisation, and NASA is exploring ways to search for such signs.
Beyond the solar system
A major project involving technosignatures today is Breakthrough Listen, a privately funded $100 million program that is searching stars and galaxies for radio signals and laser transmissions. It has been ongoing since 2016, and it was backed by Professor Stephen Hawking. Supported by Astronomer Royal Lord Martin Rees, it aims to cover ten times more of the sky than previous programs. To that end, it’s the world’s largest SETI project, making good use of the Green Bank Telescope in West Virginia and the Parkes Telescope in New South Wales, Australia.
Astronomers are also analysing the atmospheres of planets orbiting distant stars, looking for biosignatures, knowing that methane, oxygen and ozone are also the telltale signs of life. Thousands of exoplanets have been discovered since 51 Pegasi b was found in 1995 by scientists Michel Mayor and Didier Queloz, and there’s a chance that life will be found on at least one of these or others that are continuously being discovered.
Aiding this effort is the James Webb Space Telescope (JWST), which can find habitable worlds outside of the solar system and spot signs of an atmosphere like our own with greater sensitivity than ever before. Some promising candidates include K2-18b, which orbits the red dwarf K2-18, 124 light years away from Earth – JWST revealed the presence of carbon-bearing molecules, including methane and carbon dioxide, as well as dimethyl sulphide which, on Earth, is only produced by life.
The truth is out there
The TRAPPIST-1 system of seven rocky planets, some of which are in the star’s habitable zone, is also intriguing. While an analysis of the innermost planet found little evidence of an atmosphere, that doesn’t rule out the possibility it exists on others. In many ways, JWST is the game-changer, and it is potentially becoming ever more useful. A study from The University Of California, Riverside, has identified artificial greenhouse gases that would indicate a far-flung world artificially modified to allow life to flourish.
One thing’s for sure: as technology advances, so do the chances of spotting life elsewhere. It’s encouraging to see projects such as the Habitable Worlds Observatory, a large infrared, optical and ultraviolet space telescope which could be launched in the 2040s as a successor to JWST.
Specifically created to find and image Earth-size habitable exoplanets in Goldilocks zones, it’d be the first time a telescope has been designed for the sole purpose of finding signs of life on exoplanets, and it’s a sign that NASA is serious about such endeavours. After all, it’s becoming less and less likely that Earth is the only planet in the universe to host life, and it’s only a matter of time before this is proven. The truth, as they say, is out there. It’s just a matter of pinning it down.

Major Jesse A Marcel
holding debris related
to the Roswell Incident
in New Mexico.

If we find life, it’s
likely to be contained
within rocks or within
subsurface oceans.

© Adobe Stock (AI Generator); Alamy

Alien

signals

There is every chance that an advanced, intelligent alien species has either been trying to make contact with beings on other planets or is carrying out activities that send radio signals into space. Some human scientists aim to try and detect them, and there have been instances when a breakthrough has seemed possible.
On 15 August 1977, for example, the Big Ear radio telescope at Ohio State University churned out a data printout made up of a baffling sequence of numbers and letters. It read “6EQUJ5” prompting astronomer Jerry Ehman to take one look at it, circle it in red ink and write “Wow!”. This so-called Wow! Signal has never been repeated, and astronomers can’t agree on what it was. Some still believe it was sent by aliens.
Other signals have included Breakthrough Listen Candidate 1, observed in 2019 in the direction of the star Proxima Centauri, and a quickly deleted report of a narrow-band electromagnetic signal detected by China’s giant Sky Eye telescope in 2022. Mysterious brief radio bursts from deep space could also be evidence of advanced alien life, some astronomers say.

“Drake’s
equation has
become hugely
important in the
search for life”

Missions to Enceladus
and Europa could
discover life on
alien moons.

TOO HOT

Planet size
1-2x Earth

TOO COLD

JUST RIGHT

Institutions like SETI
use terrestrial and
space telescopes in
the hopes of finding
evidence of alien life.

In April 2030, Europa
Clipper will start
searching the moon
for life signs.

The star TRAPPIST-1 and
its orbiting exoplanets
are good candidates
for finding life.

THe

Golden Record

They say vinyl is timeless. Well, you’d hope so, given NASA placed records on board the Voyager 1 and Voyager 2 space probes when they were launched in 1977. Although they were actually 12-inch gold-plated copper disks, they may one day turn out to be the most important recordings ever made. After all, they’re now so many billions of miles away, that they can only ever be picked up and heard by alien civilisations – and there’s enough content on the records to give them a good flavour of life on Earth.
The hope is that extraterrestrial beings will be able to follow the sleeve instructions, use the bundled cartridge and needle and listen to a variety of natural sounds, including those made by birds, whales, surf and wind, as well as a selection of music and spoken greetings in 55 languages. They’ll be able to access 115 images, too. But if they’re inspired by all of that to pay us a visit, we’ll be waiting a long time. It’s going to take at least 40,000 years before the records reach another planetary system.

NASA’s Martian
rovers are looking at
potential ancient water
beds hoping to find
signs of primitive life.

© Adobe Stock; Getty; NASA/JPL-Caltech

Explore dazzling quasars: what they are,
how they form and the ongoing quest to
unravel their enigmatic origins and discover
the secrets they hold about the universe

Picture an object that’s so dazzlingly bright that it can outshine an entire galaxy, yet so distant its light has travelled billions of years to reach us. What you’ve just conjured up in your mind’s eye without realising it is a quasar.
These truly extraordinary objects are among the most fascinating phenomena in the known universe. First discovered by astronomers in the 1950s and 1960s, quasars challenge our understanding of the cosmos and continue to inspire astronomers as they probe deeper into their chin-scratching mysteries.
So, what exactly are quasars? How are they created? And what precisely can they tell us about the history and evolution of the universe? Let’s delve into these cosmic beacons and find out something about the continuing scientific quest to uncover their long-held secrets.
The term ‘quasar’ is short for ‘quasi-stellar radio source’, a name that reflects their star-like appearance in early observations. However, quasars aren’t stars at all. They are, instead, the luminous cores of distant galaxies powered by supermassive black holes that reside at their very centres. These black holes, often billions of times the mass of our Sun, are surrounded by ‘accretion disks’ - vast regions of gas and dust constantly spiralling inward.
As matter falls into the black hole, it heats up due to immense friction and incredibly intense gravitational forces, releasing staggering amounts of energy in the form of light and other similar sorts of radiation. This process makes quasars some of the brightest objects in the entire universe. Despite their brilliance, their vast distances, some of which are over 13 billion light years away - make them appear merely as faint little points of light in the night sky. Hence the confusion, with many assuming that they’re ‘just’ stars.
One way to think about it is that quasars are kind of like cosmic lighthouses, shedding light into the farthest reaches and earliest epochs of the universe. Their super-bright light enables astronomers to study the conditions of the cosmos as it existed billions of years ago.
Collision course
Quasars form in galaxies with supermassive black holes actively ‘accreting’ or gaining matter. When a black hole ‘feeds’, its accretion disk becomes an intense source of energy. But what triggers this bizarrely ravenous feeding frenzy?
One leading explanation on the subject involves galaxy collisions (also known as ‘mergers’). When two galaxies collide, their gravitational interactions can disrupt the orbits of stars and gas, channelling significant amounts of material toward the central black hole. This sudden influx of matter fuels the accretion disk, effectively igniting the quasar.
Another factor is the availability of cold gas, which serves as the raw material for accretion. In the early universe, galaxies were incredibly rich in this kind of gas, leading to more and more frequent and intense quasar activity. Over time, galaxies evolved and their gas supplies diminished; as a result, quasars became less common, explaining why they are now primarily observed in the distant, early universe.
Big deal
Despite remarkable progress, understanding the formation of quasars remains shrouded in no small amount of mystery. As is the case with so many things in astronomy.
One of the biggest questions that still requires answering when it comes to quasars is just how supermassive black holes grew so impressively gigantic in the days of the early universe. Observations of quasars with high redshifts – those whose light has been stretched significantly due to the universe’s expansion – suggest that some black holes had already reached masses of over a billion Suns less than a billion years after the Big Bang.
This almost unfathomably rapid growth is difficult to reconcile with current models of black hole formation and growth. Did they form from the collapse of massive primordial stars, or were they born from dense clusters of stars and gas? Alternatively, could they have grown through the direct collapse of massive gas clouds? Each hypothesis has its challenges and limitations, with researchers actively exploring all these possibilities through various simulations and observations.
Another enigma is the precise role of quasars in galaxy evolution. Quasars emit powerful jets of particles and radiation that can influence their host galaxies. These ‘outflows’ can heat and expel gas, potentially ‘quenching’ or suppressing star formation. On the other hand, though, they might also compress gas, triggering bursts of star formation. Understanding these kinds of outcomes and feedback mechanisms is crucial for piecing together the life cycles of galaxies and the exact job of quasars.
Recent advances in technology have propelled quasar research to dizzying new heights. Telescopes like the Hubble Space Telescope and the more recently launched James Webb Space Telescope (JWST) have provided unprecedented views of quasars and their host galaxies. The JWST, in particular, is designed to study infrared light, which is absolutely ideal for observing the distant, redshifted light of early quasars.
One of the most exciting discoveries in recent years is the identification of quasars at extremely high redshifts. In 2021, astronomers discovered the catchily named J0313-1806, a quasar with a redshift of 7.64, meaning its light has travelled over 13 billion years. This quasar hosts a black hole with a mass of 1.6 billion Suns, challenging existing hypotheses regarding black hole growth.
Spectroscopic studies of quasar light have also provided lots of undeniably valuable information about the intergalactic medium, the vast expanse of gas that exists between galaxies. By analysing the absorption lines in quasar spectra, scientists can map the distribution of elements like hydrogen and helium and study the ‘Reionisation Era’, a period when the first stars and galaxies ionised the universe’s neutral hydrogen.
Computer simulations have backed up these observations by modelling how quasars and their host galaxies evolve. Programs like the Illustris and EAGLE simulations have helped scientists test lots of their ideas and theories about quasar formation and feedback. Plenty of the data collected seems to align quite closely with their hypotheses. While some have, understandably, caused experts to tear up plenty of journals and wipe lots of blackboards clean.
A light in the dark
Quasars play a pivotal role in understanding the expanding universe. Their immense brightness makes them ideal markers for measuring things like cosmic distances.
These fascinating cosmic beacons are also used as probes to study the large-scale structure of the cosmos. By mapping the distribution of quasars, scientists can trace what they call the ‘cosmic web’ of matter that forms the very backbone of the universe’s structure.
Much like quasars themselves, the future of research into the area is looking very bright indeed, largely thanks to next-generation telescopes and new missions and projects. The Vera C Rubin Observatory, set to begin operations soon, will - amongst other things - conduct the Legacy Survey Of Space And Time (LSST), mapping billions of galaxies and quasars. This survey will provide an unprecedented dataset for studying quasar evolution and distribution.
Space-based missions like the Nancy Grace Roman Space Telescope will further enhance our ability to observe distant quasars. With its wide-field infrared capabilities, the Roman Telescope will complement JWST’s already very detailed observations, helping to answer questions about the early universe.
Not only that but, advancements in machine learning and artificial intelligence are enabling researchers to sift through incredibly vast amounts of data much quicker and more efficiently. In doing so, they’re able to begin identifying new quasars and uncovering patterns that were previously hidden.
Despite our earlier likening to them, quasars are far more than just cosmic lighthouses; they’re keys to understanding the universe’s history and structure. From their formation around supermassive black holes to their role in shaping galaxies and illuminating the cosmos, quasars offer invaluable insights into the workings of the universe. Yet, all sorts of deep mysteries still remain, driving astronomers to push the boundaries of technology and theory to try and solve them.
As terrestrial and space telescopes grow more powerful and our understanding deepens, quasars will continue to captivate and challenge us. These dazzling beacons not only reveal the universe’s distant past but also remind us of the boundless curiosity that fuels our exploration of what’s out there and beyond.

© Adobe Stock; NASA

The importance of

redshift

Redshift is the stretching of light’s wavelength as it travels across the always-expanding universe, shifting it toward the red end of the spectrum. It’s a cosmic Doppler effect, revealing how fast objects like galaxies and quasars move away from us.
Redshift is crucial in the context of quasars because it helps astronomers determine their distance, age and the speed at which they are receding (due to the universe’s expansion). Quasars are incredibly bright, but their extreme distances mean their light has travelled for billions of years to reach us. So, measuring their redshift provides something of a cosmic timestamp, revealing how far back in time we’re observing them.
This is particularly significant because quasars are most abundant in the early universe, serving as markers of its infancy. Their redshift allows scientists to study the conditions of the universe when it was young.

The ESA’s Euclid space
telescope is equipped
to measure the redshift
of galaxies.

The X-ray image of the
quasar PKS 1127-145,
taken by the Chandra
X-ray Observatory.

A selection of Hubble’s
survey of quasars, the bottom
row shows the observations
with the glare removed.

Quasars

Illustration showing the
light of two quasars
at the heart of two
merging galaxies.

Let’s head for the stars and discover
what it is that makes the Great Attractor
theory such a magnetic idea that we can
not help but get sucked into

the Great Attractor Theory

Imagine being pulled by an irresistible force towards something you can’t see. That’s the tantalising mystery at the heart of the Great Attractor, a gravitational anomaly influencing the movement of galaxies across hundreds of millions of light years. Hidden behind the dense plane of the Milky Way, the Great Attractor is one of the most intriguing enigmas that exists in astronomy today. Holding, as it does, a huge number of vital clues about the universe’s large-scale structure and so many of its unseen forces.
The Great Attractor is not an object in the traditional sense, so it’s crucial not to consider it as such. It makes much more sense to regard it more as a region in space – one where gravity exerts a significant pull (hence the ‘attractor’ moniker). It lies approximately 150 to 250 million light years away, within the Laniakea Supercluster, of which our Milky Way is a small component.
Galaxies, including our own, are racing towards this region at astonishing speeds of over 600 kilometres per second. Which, you’ll probably agree, is remarkably quick. However, its exact nature remains elusive due to its position in the so-called ‘Zone Of Avoidance’, where the Milky Way’s stars and dust annoyingly obscure our view.
Identifying the anomaly
The story of the Great Attractor begins in the late 1970s and early 1980s, when astronomers began studying the peculiar motion of galaxies. While galaxies generally follow the universe’s expansion, scientists working in cosmology noticed some stark deviations from this expected movement. They appeared to be drawn towards a specific region in the southern sky.
This head-scratching anomaly became the focus for a dedicated group of astronomical researchers dubbed ‘the Seven Samurai’ who were finally able, in 1987, to identify the general direction of this gravitational pull. Using redshift data – the measure of how light stretches as objects move away – they were able to discover and reveal that entire clusters of galaxies were converging towards what we now call the Great Attractor.
Despite the initial excitement surrounding the discovery, truly understanding the nature of the Great Attractor proved somewhat more challenging. Early observations were limited by the technology available at the time. The obstructive Zone Of Avoidance didn’t help, making it difficult to determine whether the anomaly was a massive galaxy cluster, an unseen cosmic structure, or something even stranger and more unknown.
The Great Attractor is crucial for understanding the large-scale structure of the universe, a cosmic and unknowable web of galaxies and voids held together by gravity. It helps astronomers map how galaxies are distributed and how they move within this incredibly immense and complex structure.
The pull of the Great Attractor also provides astounding insights into the forces that shape the cosmos. While its exact cause remains uncertain, its influence underscores the incredible importance of dark matter – the invisible and enigmatic substance thought to make up around 85% of the universe’s mass – and the interplay of gravitational forces which exist on a seriously breathtaking scale.
The Great Attractor also highlights the interconnectedness of the Laniakea Supercluster, revealing that even our seemingly isolated Milky Way is part of a much more significant and super-dynamic system. Understanding this relationship could have profound implications for astronomers who are striving to find out more about our place in the universe.
While mainstream explanations suggest that it’s a dense cluster of galaxies obscured by the Milky Way’s dust and gas, more speculative theories take a wilder turn. Some theorists propose that the Great Attractor might be a tear in space-time, where alternate realities converge – or even the product of an advanced extraterrestrial civilisation which is harnessing unimaginable forces to do its own unknown bidding.
Sucked in
The most outlandish, fun and – admittedly – unlikely idea posits that a sentient, purposeful force – essentially a cosmic tractor beam – is guiding us toward an unknown destination. Which is, of course, fun to think about, terrifying to imagine and rather unlikely to be true. Though it would make for some good science fiction.
The Great Attractor is a giant source of persistent scientific curiosity and debate – mostly because there’s such a vast array of information we don’t know about it. Key unanswered questions include…
What exactly is it? Is the Great Attractor a massive galaxy cluster, a supercluster, or a complex structure involving dark matter?
How does it relate to larger cosmic structures? Recent studies suggest that the Shapley Supercluster, an extremely massive collection of galaxies farther away, could also influence the motion of galaxies in our region, possibly even overshadowing the Great Attractor’s role.
What role does dark matter play? The gravitational forces observed could be partially explained by dark matter, but its distribution and properties remain elusive.
What don’t we see? Could there be phenomena beyond our current understanding of physics, such as undiscovered particles or forces influencing this region?
Over the years, several theories have emerged to explain the Great Attractor’s gravitational pull, such as…
Galaxy clusters: observations suggest that the Great Attractor could be centred on a massive galaxy cluster, like the Norma Cluster, which is rich in all sorts of galaxies and vast swathes of dark matter.
Larger cosmic structures: some scientists and experts argue that the Great Attractor is part of a broader network of superclusters, including the Shapley Supercluster. This theory, as we’ve mentioned, has it that the gravitational pull is not from a single source but a combination of massive structures.
Dark matter dominance: given that the amount of visible matter falls somewhat short of explaining the gravitational pull that’s so far been observed, dark matter likely plays a pretty significant role. Its precise contribution, however, is still under some investigation.
Unknown physics: some researchers speculate that new physics - things that are beyond our current understanding - could be at play, such as modifications to gravitational theory or the effects of dark energy in ways not yet understood. Or, of course, forces we’ve not even begun considering or understanding yet.
Attracting attention
The quest to understand the Great Attractor continues, driven by advances in technology and new research initiatives. Modern telescopes, such as the James Webb Space Telescope (JWST) and the Square Kilometre Array (SKA), promise to enhance our ability as a species to observe obscured regions of the universe and detect faint signals from distant structures.
In addition to that, simulations of the universe’s evolution, using powerful supercomputers, are helping scientists model the gravitational interactions between galaxies and cosmic structures. These models can provide insights into how anomalies like the Great Attractor might form and evolve over time.
Finally, studies of dark matter and dark energy are critical. By probing these sorts of elusive phenomena, researchers hope to refine our understanding of the forces that help shape the universe, including the enigmatic pull of the Great Attractor.
The Great Attractor is a testament to the universe’s almost laughable complexity and also the very limits of our human understanding. Hidden behind the Milky Way’s dusty veil, it exerts a gravitational pull on galaxies across vast distances, challenging astronomers to unravel its many, many secrets. While advances in technology and theory have provided tantalising glimpses of this cosmic anomaly, so much remains to be discovered.
In many ways, the Great Attractor serves as a reminder of humanity’s enduring curiosity and drive to explore. As we peer deeper into the universe and refine our understanding of its workings, the answers we find may not only illuminate the Great Attractor’s nature but also shape - or potentially reshape - our understanding of the cosmos itself. The pull of discovery, like the Great Attractor’s gravitational force, is impossible to resist.

The Milky Way creates
a Zone Of Avoidance –
obscuring our vision of
the Great Attractor.

The Norma Cluster is
crammed with galaxies and
located near the centre
of the Great Attractor

Map of the Laniakea
Supercluster. Our
home system is
coloured blue.

The Zone Of Avoidance

With its moniker not unlike a term you might expect to hear from a psychiatrist while lying back on a leather sofa, the ‘Zone Of Avoidance’ (sometimes known as ‘ZOA’) is a patch of the sky where galaxies hide from view. At least, that’s what they try to do, anyway.
This cosmic blind spot occurs because the dense plane of the Milky Way is incredibly crammed with so many stars, gas and dust that it blocks our telescopes from seeing beyond them all. It’s a little bit like trying to spot a distant lodge through a dense forest. For decades, astronomers thought this region was a true void, but advancements in infrared and radio telescopes have revealed a whole bunch of galaxies lurking behind the obscuring dust curtain.
Why does it matter? Well, the ZOA spans a significant portion of the sky and potentially conceals some rather fascinating clues about the universe’s large-scale structure. Unlocking its secrets could change how we understand cosmic evolution itself. So, while the name might suggest something of a barren zone, it’s more like a hidden treasure map waiting to be explored. If only we could sneak a peek…

Our universe is
incredibly dense,
making studying its
finer points difficult.

© Adobe Stock; Getty; ESA; NASA; Andrew Z Colvin

Collisions between galaxies provide
some of the most breathtaking of all
astronomical images – but they play an
important role in galactic evolution too

Collisions can cause
‘tails’ to appear, which
make for spectacular
visuals for observers.

When Galaxies collide

Stars virtually never collide with each other because they’re enormously far apart compared to their size. For example, the distance to the Sun’s nearest neighbour is 30 million times its diameter. But the same isn’t true of galaxies. The Milky Way’s closest neighbour, the Andromeda galaxy, is a mere 25 times further away than the Milky Way’s diameter. So collisions between galaxies can and do occur on a fairly regular basis.
Although the universe is expanding, most galaxies occur in groups or clusters – and random motions within these mean that galaxies are just as likely to approach each other as to recede. Once they start to get closer, they’re pulled further together by their mutual gravitation, with the inevitable result being a ‘galaxy collision’. But this isn’t the disastrous cosmic car crash the name might suggest. Galaxies are mostly empty space, so it’s more accurate to picture them passing through each other rather than colliding. This all takes place on a glacially slow timescale, over hundreds of millions of years, so any observers inside a colliding galaxy would most likely be blissfully unaware of it.
Slow motion
Given the slowness of galaxy collisions, astronomers can’t observe them unfolding in the sky. All that is visible is a frozen snapshot of a particular stage in the process. Fortunately, however, there are so many colliding galaxies in the universe – all seen at different stages and from various angles – that by looking at all of them, it’s possible to piece together a picture of the entire process. This observational data can then be coupled with computer simulations, which have the effects of gravity and other basic physics built into them, to provide a reasonable understanding of the science of galaxy collisions.
One fact that emerges is that galaxies don’t have to literally collide to show significant effects. Even if they’re not in actual physical contact, the mutual gravitation of a pair of galaxies can dominate their visual appearance. This can be seen in one of the most familiar of nearby galaxies – the Whirlpool Galaxy, much loved by amateur astronomers because it’s easily visible through a small telescope. It’s actually two separate objects – a large, elegant spiral galaxy with a smaller dwarf companion connected to one of its spiral arms. This is an example of an ‘interacting pair’ of galaxies – a stage that often precedes an actual collision or galactic merger.
Other interacting pairs of galaxies can be dramatically distorted by the interaction. One common occurrence is the presence of long, narrow tails, seen, for example, in the evocatively named Tadpole, Mice and Antennae galaxies. Commonly referred to as ‘tidal tails’, these galactic appendages are formed by a physical process analogous to the way the Moon raises tides in the Earth’s ocean. The latter occurs because the Moon’s gravitational pull is stronger on one side of the Earth than the other. In the same way – but on a much vaster scale – the pull that one galaxy of an interacting pair exerts on the other varies across the width of the galaxy. This can drastically distort the galaxy’s shape, flinging stars and gas onto completely different orbits. Tidal tails are perhaps the most visually spectacular result of this process.
Lucky stars
Although stars are the most visible component of galaxies, they often contain large clouds of gas as well – and these can also be affected when pairs of galaxies interact gravitationally. It’s from these gas clouds that stars are formed, but only if the density of gas is high enough – which it may not be if the gas molecules are evenly distributed throughout the galaxy. But the tidal effects of another nearby galaxy can compress the gas to the point of triggering a new burst of star formation, even in galaxies that ceased normal star formation long ago. Newly formed stars tend to be very bright, so regions in which star formation is active can be particularly eye-catching – another reason why interacting galaxies are so popular with astrophotographers!
But colliding and merging galaxies aren’t just a visual oddity of the universe – they play a fundamental role in galactic evolution. For the most part, the first galaxies to form not long after the Big Bang were systematically smaller than present-day ones. The main processes driving the growth of galaxies to their current proportions have been mergers between similar-sized galaxies and the ‘cannibalism’ of dwarf galaxies by larger ones. Astronomers have found plentiful evidence of the latter process in our own Milky Way because material torn out of a cannibalised dwarf gets drawn out into a long ‘stellar stream’ that still remains discernible after billions of years. By observing streams of this type, astronomers have identified six different merger events in the Milky Way’s history.
In some cases, while losing many of its stars in the encounter, a dwarf galaxy may survive a collision and continue to orbit around the Milky Way – possibly colliding with it again in the future. A notorious repeat offender of this type is the Sagittarius dwarf galaxy, which has rammed its way through the Milky Way’s disc on three separate occasions, around 5,7, 1.9 and 1 billion years ago. And it doesn’t stop there – according to computer predictions based on its current motion, Sagittarius is due for another collision around a hundred million years from now.
Dwarf galaxies aside, there’s a much bigger galactic collision in the offing. By extrapolating their current velocities, astronomers have established that the Milky Way and its slightly larger neighbour, Andromeda, are on a collision course with each other. Based on precision measurements of stellar motions made with the European Space Agency’s Gaia telescope, the best estimate of when the crash is going to occur is around 4.5 billion years from now. That’s roughly the same as the present age of the Earth, so it’s safe to say there’s no immediate cause for alarm. But as we’ve seen already, there wouldn’t be anyway because galactic collisions tend to be slow, sedate affairs unlikely to disrupt individual stars or planetary systems. On the other hand, the burst of star formation that’s likely to accompany the collision would provide some spectacular night-sky views for any inhabitants of Earth that are still around when it happens.
Although the collision is such a long way in the future, the mathematically predictable nature of gravity means that detailed computer simulations can give us an idea of what will happen. After the initial encounter, the two galaxies will bounce around their common centre of gravity for two billion years or so. During this time, the solar system may find itself thrown way out to large distances in a tidal tail. But apart from being a dramatic place from which to view the goings on, this shouldn’t be dangerous as the Sun and its retinue of planets are expected to remain unscathed.
Meet Milkomeda
After the dust settles, the result of the collision will almost certainly be a single giant elliptical galaxy containing most of the mass previously contained in both the Milky Way and Andromeda (astronomers have coined the fanciful name ‘Milkomeda’ for this combined giant). Elliptical galaxies are less dramatic-looking than spirals – largely featureless blobs shaped roughly like rugby balls, composed mainly of relatively old stars with little gas or active star formation. Many of the largest ellipticals are believed to be the result of mergers between two or more spirals, and indeed, this does seem to be the most likely consequence of the way these encounters unfold. Stellar orbits become randomised rather than remaining in flattened discs, which results in the characteristic elliptical shape. At the same time, the compression of gas during the collision triggers such an intense burst of star formation that very little gas is left over for further star formation after that.

A New Look at

Colliding Galaxies

Colliding, merging and interacting galaxies are a relatively common sight in the universe, and they’ve always captivated astronomers and the public alike. Many of the most iconic photographs taken by NASA’s Hubble telescope have featured them. However, Hubble’s successor, the James Webb Space Telescope (JWST), is shedding a whole new light on such spectacles. To start with, rather than being limited to the visible band, it’s optimised for infrared wavelengths – the best way to observe the massive bursts of star formation that often accompany galactic mergers. The unprecedented power of JWST also allows it to pick out distant galaxies, which – due to the time it took their light to reach us – we see as they were not long after they formed. Even in those primordial times, galaxies had already begun to merge with each other – and JWST is the first telescope to capture images of this happening.

Timeline displaying
the Sagittarius
galaxy’s interactions
with the Milky Way.

The Milky Way and
Andromeda will
collide and merge in
4.5 million years.

“collisions tend
to be slow, sedate
affairs unlikely
to disrupt
individual stars”

104

Is humanity alone, or are there civilisations
on extrasolar planets waiting for contact?

88 Exoplanets
94 rogue planets
98 the search for alien life
104 The fermi paradox

The hunt for worlds around other stars is three
decades old. What do we know so far about these
alien planets, and what might we find in the future?

For almost the entirety of human history, our knowledge of worlds among the stars has been confined to one location, our solar system, a string of planets orbiting a single star. We slowly came to understand some of the other planets around our Sun and their moons, but our expanse into the cosmos was fleeting. In terms of rocky habitable planets, there was just one: Earth.
That all changed in the 1990s when astronomers, for the first time, discovered planets orbiting other stars. Of course, we had long expected there to be such planets, known as exoplanets, given there are up to 400 billion stars in the galaxy, but finding them was incredibly difficult. They were small, tiny against their star’s glare, and undetectable by most telescopes. What hope did we have of finding and studying these worlds in earnest?
Following the evidence
In 1992, astronomers discovered the first exoplanets orbiting an unusual star, a pulsar, a rapidly spinning neutron star – the dense core left behind after some stars explode as supernovae. The planets were found because of the slight gravitational pull they had on the pulsar, causing it to wobble ever so slightly as it spun, a technique called the radial velocity method. The two planets were found to be a few times more massive than Earth on orbits similar to Mercury’s around our Sun, but not much more was known.
A similar system to our own was found for the first time in 1995 when astronomers discovered a planet orbiting the Sun-like star 51 Pegasi about 50 light years away from Earth. Larger than Jupiter but on a tight orbit, 20 times closer to its star than Earth orbits the Sun, the world became the first known example of a ‘hot Jupiter’, a gas giant planet at extremely hot temperatures because of its proximity to its star. The discovery was a huge breakthrough, proving other stars like our Sun could host planets. The hunt was on for worlds like our own.
The planet hunter
A trickle of exoplanets followed over the subsequent decade, but the hunt was transformed in 2009 with the launch of NASA’s Kepler Space Telescope. This telescope was designed to hunt for planets using the transit method, looking for the dip in a star’s light as an exoplanet passed in front from our point of view. The strategy required a bit of luck – you needed a planet’s orbit around its star to be lined up for us such that it passed in front of its sun – but the thinking was that if we looked at enough stars, there would be a decent fraction that hosted transiting planets, allowing us to find and study exoplanets.
That bet paid off enormously. Kepler exploded the field of exoplanet science, finding nearly 3,000 exoplanets through observations of more than half a million stars in the Milky Way. By the time it was retired in 2018, Kepler’s planets constituted the vast majority of known alien worlds around other stars, and they still make up nearly half of the 6,000 known planets today. Other telescopes have followed in Kepler’s footsteps – including NASA’s Transiting Exoplanet Survey Satellite (TESS), launched in 2018, and ESA’s Characterising Exoplanets Satellite (CHEOPS), launched in 2019.
Kepler’s discoveries showed us that there was a wide variety of planets in the universe, somewhat unlike the diversity in our own solar system. The telescope was better suited to finding planets that caused a significant transit, or dip, in their star’s light, which tended to be larger gas giant worlds on tight orbits. But it also found rocky planets orbiting low-mass stars, red dwarfs, the most common type of star in our galaxy, and strange worlds between Earth and Jupiter in size, thought to be either super-Earths or sub-Neptunes.
One of its key discoveries was Kepler-22b in 2011, the first known exoplanet to be orbiting in a star’s habitable zone, the region from a star where conditions are just right for water to form (and where Earth resides). Although twice the size of Earth, the discovery showed that the existence of planets in habitable zones was not something that just happened in our own solar system, and it raised the prospect that there might be rocky planets similar in size to our own planet orbiting in the habitable zone of other Sun-like stars – prime locations to look for life.
Familiar worlds
Finding these ‘Earth 2.0’ planets has been difficult because Sun-like stars tend to be quite bright, and it is hard to spot terrestrial Earth-sized worlds orbiting them on similar orbits to our own planet. But we have been able to find many rocky planets orbiting smaller red dwarf stars because the glare of the stars is significantly less. One of the most famous systems is TRAPPIST-1, which was discovered in 2016 by the La Silla Observatory in Chile. Located about 40 light years from us, this system is exceptional because it was found to have seven rocky planets all orbiting the star, with up to four possibly being in the star’s habitable zone.
One vital unanswered question is how hospitable red dwarfs are for life. These stars are much more active than our own and prone to frequent flaring events, which can irradiate planets and blast off their atmospheres. Even with exciting systems like TRAPPIST-1, it was unclear whether we might expect to find life in such systems. Most of our exoplanet studies up to that point just gave us basic information on the planets – their size and their orbit around the star – without much more detail. How could we know which planets were habitable or not?
To find out, powerful telescopes like NASA’s Hubble Space Telescope and James Webb Space Telescope (JWST) have been used to look very closely at some exoplanets with their large mirrors. Watching the planet as it transits in front of the star, they can detect the star’s light passing through the planet and work out if there is an atmosphere present and what gases are in the atmosphere. This has allowed us to make tentative studies of some exoplanets, mostly limited to larger gas giant worlds where the atmospheres are easier to detect. Through this method, we have discovered water, carbon dioxide, and more in the atmospheres of many planets.
Cutting-edge telescopes
To advance our understanding of exoplanets, we need bigger and better telescopes that can more easily pick apart their atmospheres. Those are in development right now, and none is more exciting than NASA’s upcoming Habitable Worlds Observatory (HWO). Set to launch in the 2040s, the telescope is designed with one key goal in mind: finding and taking images of planets exactly like Earth orbiting in the habitable zones of nearby Sun-like stars. These images, although likely to be small blobs of light, could allow us to pick apart the light of the planet, perhaps telling us what its atmosphere is made of. Looking for chemical imbalances in the atmosphere, we could also look for signs that there might be life on the surface expelling gases upwards, like how short-lived methane is produced by animals on Earth. In the most optimistic scenarios, the telescope could even see oceans and continents on some worlds, and perhaps even night lights, if a planet were to be home to an intelligent alien civilisation – a slim but tantalising possibility.
Water worlds
It is not just Earth-like exoplanets that are of interest, though. Some worlds might be ocean worlds, planets almost entirely encompassed in water. These worlds could be fascinating places to look for life, although it is thought that having some land is imperative for evolution. Nonetheless, astronomers have found some planets that could have large amounts of water and might be hypothesised water worlds, fascinating targets of study. Other planets might be frozen snowballs, scorched Earths, or even worlds on the border of being rocky planets and gas giants.
The fun doesn’t stop there. While there are eight major planets in our solar system, there are many, many more moons, numbering in the hundreds, orbiting these planets. Lots of these moons are small asteroid-like bodies, but some, like Jupiter’s moon Europa and Saturn’s moon Titan, are large, interesting places to look for life. Could exoplanets have moons, too, known as exomoons? That is something astronomers have been investigating, looking for evidence of large satellites orbiting planets around other stars.
Early searches have focused on massive moons, maybe as big as Earth or more, around gas giants. If we can find such worlds, they might be prime locations to look for life. Gas giants could orbit in the habitable zones of stars, or moons around some of these exoplanets could be kept warm and wet by the heat from their host planet. It might be that exomoons are the predominant place that life forms in the universe, given how numerous they are compared to planets, so finding and studying some of these moons is crucial for our search for life. Many of these moons could be icy locations, too, like Europa. Perhaps the majority of organisms in the universe aren’t surface-dwellers like us on Earth but subsurface creatures hidden beneath view under alien ice sheets, never to see the stars.
Signs of life
If we can find exoplanets and exomoons that appear to be habitable, then perhaps we could start to look for signs of life in earnest. That could include looking for intelligent life and trying to find signals from other intelligent civilisations either purposefully or accidentally sent out into the universe. Those searches have so far been fruitless, including in systems like TRAPPIST-1, but as more and more potentially habitable solar systems are found, we will be able to refine our hunt and focus on worlds that seem to have all the right conditions for life. Perhaps one day, we might even be able to hold conversations with intelligent alien civilisations, sending back-and-forth messages over the vast distance of the galaxy on inhabited exoplanets.
Actually, visiting an exoplanet is a much more daunting prospect, given the vast distances involved. Even our closest star, Proxima Centauri, is some 4.2 light-years from Earth, about 9,000 times farther than the planet Neptune. A voyage there with current technologies would take many millennia, meaning any hope of reaching exoplanets in a reasonable time will require a novel breakthrough in spacecraft propulsion. Perhaps more likely is that we might find a way to accelerate a small probe to immense speeds, such as using the outward radiation from our Sun to propel a solar sail and take a snapshot of an exoplanet as the probe whizzes past. By some calculations, such a mission could be possible in decades; perhaps by the end of the century, we will have our first-ever true close-up image of an exoplanet.
Until then, we are continuing to explore the worlds of our galaxy, alongside the occasional discovery of a planet in galaxies beyond. What is becoming clear is that there is a wide diversity of planets and solar systems awaiting us in the cosmos, some like our own, and some vastly different. Which of these plays host to life like our own solar system, if any, remains an open question. But with every passing year, our knowledge of exoplanets increases more and more. We may be alone, we may not, but we are certainly one of many fascinating worlds in the universe.

Kepler 22b was the first
discovered exoplanet to
lie within a star’s
habitable zone.

Concept of the upcoming
Habitable Worlds
Observatory, slated to
launch in the 2040s.

Proxima Centauri b

Proxima Centauri is the closest star to Earth, a red dwarf star about 4.2 light-years away. In 2016, an exciting discovery was made of a rocky planet orbiting the star, called Proxima Centauri b, within the habitable zone of the star. At least two other planets are also thought to orbit the star as well.
The planet completes an orbit every 11 days (the habitable zones of red dwarfs are much smaller than Sun-like stars), and it is similar in size to Earth. If the planet has an atmosphere, which is not known yet, it could have the right temperature and conditions to support liquid water on its surface and perhaps even life. However, the planet receives much more radiation from its star than we do from our Sun, dampening prospects of life, although not completely ruling them out. Studies and observations of the planet continue today in earnest.

Since the first discovery
in the 1990s, thousands
of exoplanets have
been catalogued.

Kepler was a virtuosic
planet-finder,
discovering almost 3,000
alien worlds.

© Adobe Stock; Getty; NASA; ESO

Hot jupiters

The most common type of planet discovered so far in our galaxy are hot Jupiters. These are gas giant planets on very tight orbits around stars, typically less than ten days but sometimes completing an orbit in mere hours. This proximity gives the planets extreme temperatures of thousands of degrees Celsius, making them not particularly suitable places to look for life.
How these planets ended up in such tight orbits is an open question. It’s difficult to form such large planets so close to a star, so it might be that they formed elsewhere in their solar system before being kicked inwards, perhaps by a passing star, and migrating closer to their host star. Jupiter is also thought to have changed position early in our own solar system, although not migrating as close to the Sun, so understanding hot Jupiter formation could tell us more about our origins, too.

Exoplanets come in
many shapes and sizes,
some being water worlds
or scorched wastelands.

“we might
find a way to
accelerate a
small probe to
immense speeds”

Exoplanets

Up to four planets in
the TRAPPIST-1 system
lie within its star’s
habitable zone.

Not all planets are tethered to a star.
Some are wandering freely as orphans
and astronomers are keen to discover
more about them

To go rogue is to begin to behave in an independent or controlled way that is not authorised, normal or expected. When discussing rogue planets, that description is pretty much spot on since it describes worlds that don’t orbit their parent stars but instead drift away to wander the universe alone.
Such untethered planets are not uncommon. Isolated planetary-mass objects (iPMOs) were first discovered in the Orion Nebula in 2000, and many hundreds have been discovered since. Some astronomers now believe there are potentially trillions of these objects in our galaxy alone, outnumbering the planets that orbit stars by 20 to one (according to an analysis headed by David Bennet at NASA’s Goddard Space Flight Center). Yet, no one quite understands how they came to be.
There are two primary theories. The first is that rogue planets were once part of a planetary system but were ejected somehow, perhaps early on in their formation or later, when a dominant planet effectively booted out the lower mass worlds. This would hold for rogue planets with masses on the same level as Earth. The second hypothesis suggests they have formed alone from giant clouds of dust and gas, just like stars, but have collapsed to form a central planet surrounded by moons. In this instance, the rogue planets would be of Jupiter mass and would never have had a host star.
“Rogue planets evoke this idea of a planet ejected by a solar system, and it suggests they encircled a star at some point in the past but were kicked out for some reason,” says Dr Alexander Scholz, an observational astronomer from the University Of St Andrews in Scotland.
“But in reality, when we find objects that are in isolation, roaming through the galaxy, we don’t know how they formed and where they have come from. If you find something with the mass of Jupiter free-floating in space, it could have formed like a very tiny star in principle, but it could also have been ejected. Finding out which is which and discovering where objects are coming from is one of the big, open problems.”
Star power
Rogue planets being ejected from a star system is entirely plausible. “They are ejected from their original system based on dynamical encounters either with other planets or with other stars,” Dr Scholz explains. “Stars usually form in clusters and form their own planetary system within a few million years. Because it’s a cluster, there are other stars around, so another star might cross through that planetary system or pass by a planetary system, and the gravitation from that second star might cause instabilities that cause an ejection to happen.
“You can also have planets interacting with each other. We know from the solar system that in the early phases, planets moved around a lot, so planet migration and planets switching places is quite common, and it’s something that must happen quite a lot. During these processes, planets might encounter each other, get closer to each other, and you might get dynamical interactions, and that can also lead to planets getting kicked out.”
However they came to be, the upshot is that we likely have an abundance of these lonely, free-floating exoplanets, the majority of them dark, frozen worlds given their lack of ties to a parent star. There is plenty of evidence of them. The first rogue planet outside a star-forming region, for instance, stopped astronomers dead in their tracks. Named CFBDSIR2149-0403 and just 130 light years away, it was discovered in 2012 by astronomers using the European Southern Observatory’s Very Large Telescope and the Canada-France-Hawaii Telescope. Measuring between four and seven Jupiter masses, it’s said to be between 50 and 120 million years old with a temperature of around 400°C.
The fact it was discovered was significant in itself because while they’re believed to be plentiful, identifying rogue planets isn’t easy. If large enough, they can resemble failed stars. A good example of this relates to six rogue worlds discovered in the summer of 2024 using NASA’s James Webb Space Telescope, each between five and ten times the size of Jupiter. Given that brown dwarfs are also included in the iPMO classification and have masses from 12 times the mass of Jupiter to a star’s lower limit, you can see how rogue planets can straddle the boundary between planet and star. It doesn’t help either that both brown dwarfs and rogue planets are difficult to study because they’re less bright than stars.
So, how are rogue planets detected? “We take deep images in the infrared of star-forming regions of open clusters and try to find the faintest members of these clusters,” Dr Scholz explains. “We have found hundreds of young brown dwarfs, but if you go further and further down in mass, at some point, you also find objects that have masses comparable to giant planets.
Mass effect
“We then try to confirm them with spectroscopy. A lot of the faint little dots in those images will be other stuff in the galaxy, but some of them will have the characteristic signatures of planetary mass objects. The only way to measure these masses is to estimate temperatures or luminosities and then use that to estimate the mass by adopting a model.
“If you take a spectrum of planetary mass objects in a young open cluster, you will find that they have deep absorption features from water and methane – those features are not found in any background star. So once you have a spectrum, identifying them and making sure that they actually have a very low mass is quite easy, and this is what people typically do.”
This works well for larger rogues but less so for smaller planets, which are typically frozen due to a lack of heat from a star. “Direct imaging is not really going to work on these objects because they are so faint that they don’t emit any light that we can detect,” Dr Scholz explains.
Instead, astronomers use an alternative technique called gravitational microlensing to find objects that are comparable in mass to Earth, Mars or even Uranus. It relies on a rogue planet passing directly in front of a distant star, warping the object’s gravity. “Rogue planets act as a gravitational lens on the light that is coming from the background,” Dr Scholz says of the light from the star being gravitationally lensed and magnified, making it stand out. This enables objects to be discovered based on their mass. “That has other problems, though. You’re looking for a very singular event,” he adds.
But what is the fate of these wandering worlds? Are they doomed to be forever alone? It is possible that a rogue planet could find a new family, reeled into a solar system by the host stars’ gravity. Our own Sun may have even done this. A study in 2024 found two planets with a Mars-like mass, or three to five with a mass similar to Mercury’s, may have been brought into the solar system. They could now be residing some 1,400 times the distance between the Earth and the Sun.
We don’t need to worry, though. Most rogue worlds are said to be small – around the size of Earth – rather than of the mass of Jupiter, and that means the risk of any of these planets causing mass disruption to our solar system is slight. In any case, we’re sure to discover more insights into rogue planets in the years ahead.
The Galactic Bulge Time-Domain Survey by the highly sensitive Nancy Grace Roman Space Telescope is likely to discover hundreds of rogue worlds using microlensing, and the James Webb Space Telescope is also likely to observe many, too. “The JWST works beautifully, and it’s a fantastic tool to explore rogue planets,” Dr Scholz says. “It’s really, really primed for this kind of work.”

© Getty; Adobe Stock; Shutterstock

“Rogue planets
act as a
gravitational
lens on the light”

Some rogue planets
could have formed
independently from a
cloud of dust and gas.

Some scientists
theorise that our
solar system could have
ejected planets.

Detected

Rogue planets

Interstellar wonderers caught on camera

PSO J318.5-22 Directly observed in 2013 in images taken by the Pan-STARRS PS1 wide-field telescope, this planet-like object is eight times the mass of Jupiter. Cloudy, hot and showered with molten iron, and with temperatures inside its cloud exceeding 800°C, it is 80 light years away, and scientists believe it is just 20 million years old.
OTS 44
Discovered in 1998 and straddling the boundary between a planet and a brown dwarf, OTS 44 is in the constellation Chamaeleon, some 530 light years away. Detected by direct imaging and approximately 12 times the mass of Jupiter, it is ringed by a disc of planet-forming material that’s at least ten Earth masses.
JuMBOs
The James Webb Space Telescope has discovered hundreds of planetary-mass objects roughly the size of Jupiter, freely floating in space. A few dozen of them have been found to be moving in pairs, which has astonished astronomers, and they’ve been called Jupiter Mass Binary Objects, or JuMBOs for short.

© NASA/JPL-Caltech; Adobe Stock; Getty; Shutterstock

Our planet is one of many, yet we have
never found alien life. Where is everybody,
and does that suggest we are alone, being
watched, or something else?

The Fermi Paradox

When we look at the night sky, we see a breathtaking, glittering array of stars spreading out into the cosmos. Some twinkle with known planets, others keep their secrets hidden, but there is little doubt of the vast population of worlds above us. Our Milky Way galaxy alone, one of the trillions of galaxies in the universe, houses somewhere up to 400 billion stars, most of which will have their own planets.
That begs a bit of a perplexing question: where is everyone? If there are so many stars and planets in the universe, surely life must have arisen in multiple locations. If it didn’t, wouldn’t it stand to reason that intelligent life had developed before us? And given that we exist about 13.8 billion years after the Big Bang, one might expect there has been plenty of time for other civilisations to develop far beyond our current means.
This very question was posed by the Italian-American physicist Enrico Fermi in 1950. During lunch at the Los Alamos National Laboratory in New Mexico – so the story goes – Fermi asked his colleagues, “Where is everybody?” in reference to the perceived lack of extraterrestrials visiting Earth. He later calculated the likelihood of other Earth-like planets in the universe, the probability of intelligent life on such worlds, and the development of powerful technologies. American astrophysicist Frank Drake further developed this idea in 1961, devising a now-famous equation called the Drake Equation that attempted to calculate the probability of other life in the universe. The equation includes the fraction of stars with planets in the galaxy, the fraction of those where organic and intelligent life can appear, and the length of time that a civilisation might exist on a planet.
Pondering the paradox
Fermi concluded that we should have been visited at some point, but we hadn’t. This conundrum became known as the Fermi Paradox, and today, it remains an unsolved question about the universe. If a civilisation progresses as we have done, then theoretically, in a few million years, it should have the ability to traverse the galaxy. Why do we have no evidence this is the case? There are many possible solutions, ranging from the mundane to the frightening, as to why we never appear to have been visited by an intelligent alien civilisation. We might never know which answer is truly correct, but perhaps we will be able to rule some out one by one as we continue our studies of the heavens.
One possible solution to the Fermi Paradox is that we are alone. Perhaps, despite the vast number of stars and planets in the universe, we truly are the only world on which intelligent life – or even any life – has developed. This premise perhaps seems somewhat unlikely, but for the time being, we have no evidence of life on any other world, even in our own solar system. However unnerving, the prospect that we are alone, or at least so remote from other life that we have no hope of ever contacting it, is possible.
That latter point is another solution that civilisations have developed multiple times in the universe, but we are simply too thinly spread out either in distance or in time to ever contact one another. Journeying to our closest star, Proxima Centauri, would take tens of thousands of years with current rocket technology, and even at the speed of light, it would take four years, being four light years away. Our galaxy itself is 100,000 light years across, and our closest galaxy, Andromeda, is two million light years away. Maybe the space between inhabited planets is simply too vast for interstellar visits ever to be possible, meaning we will be limited to only ever remotely observing other inhabited worlds from afar, perhaps with the odd message of communication – and even that’s wishful thinking.
It’s possible that we clearly haven’t searched enough planets yet to know for sure if we are alone or not. Perhaps alien civilisations are sending signals to us or talking to each other, but we haven’t managed to intercept any of those signals yet. Another idea, the zoo hypothesis, suggests that such civilisations are common, but they choose not to contact us for the time being until we have reached a point of technological maturity. Maybe they are watching our world, letting us develop, before choosing to make themselves known. A variant of the zoo hypothesis is that we are living in a simulated universe, created either by an intelligent alien species or perhaps a future version of humanity, one in which the existence of aliens is kept beyond our reach.
The Great Filter
More unnervingly, perhaps civilisations often rise and fall on planets, but they cause their own extinction through war or malpractice, an idea known as the Great Filter. On Earth, we have been on the brink of nuclear war before and will likely be again. As a civilisation develops, it appears to grow the ability to cause vast amounts of destruction and coupled with unstable societies, this might be the perfect recipe for disaster. Maybe there is some point in the evolution of most civilisations in which they cause their own doom.
Most disconcerting of all, maybe we are truly alone. We know the formation of life on Earth was strenuous and a bit of a fluke. Our planet is fortunate enough to be the right size and the perfect distance from a Sun-like star to support liquid water and life. We have a magnetic field and atmosphere that is just right to protect the surface of the planet from harsh radiation, while other planets in our solar system, such as Jupiter, prevent too many asteroids and comets from crashing into our planet and ending all life. Maybe the chances of a planet having all of these requirements are slim.
Going it alone
Perhaps as a result, we are rare, alone, or even the first planet ever to achieve a habitable state. Indeed, in the grand scheme of the universe, we are relatively young. While the universe today is 13.8 billion years old, star formation is expected to continue for 1,000 billion years, or a trillion years. Relatively speaking, we have arisen quite early in the universe. Could it be that we are purely well-placed to become one of the first species to explore the cosmos? Perhaps for other worlds to come, we will be the solution to their own Fermi Paradox. Maybe the existence of humanity in the sky will prove to them that they are not alone.
For the time being, our search for life on other worlds continues. We are getting better at finding potentially habitable planets, and our methods of studying those worlds are improving. In our own solar system, the search for life continues on worlds such as Mars and Jupiter’s moon Europa. If we can find a second genesis of life, either close to home or further afield, that would be proof that Earth is not an anomaly, showing us that life can arise in multiple environments. This remains the most alluring solution to the Fermi Paradox, especially given that our study of planets around other stars is in its infancy.
However, in decades to come, if other life still appears to be non-existent, we might have to consider one of the different solutions to the paradox. Maybe we are alone, or doomed to isolation because of the vast distances of interstellar space, or watched by an omnipotent presence far above us. Just in case, let’s wish a warm hello to our alien overlords. Please don’t switch off our simulation any time soon.

Interstellar visitors

No evidence of alien life has ever been discovered in our solar system or beyond. However, we have seen evidence that objects can travel between stars, proof that it might be possible – over long time scales – to make a journey to distant planets.
In 2017, astronomers detected a cigar-shaped object flying through our solar system. Dubbed ‘Oumuamua, they surmised that it was an asteroid or comet that had originated around another star because of the vast speed it was flying, suggesting some objects like this can be ejected from stars into space and reach other stars. Another interstellar object was found two years later, the comet 2I/Borisov. While there was some suggestion these objects could be alien in origin, the much more likely prospect is they are naturally produced asteroids or comets, still an exciting possibility that hints at the prospect of interstellar travel.

At one point it was
speculated that
‘Oumuamua was an
alien spacecraft.

Proxima Centauri,
the closest star and
planetary system to us
is four light years away.

Many theorise that
civilisations could be
spread too distantly
to communicate.

“the formation
of life on Earth
was strenuous
and a bit
of a fluke”

Enrico Fermi was also
one of the physicists
working on the
Manhattan Project.

Decode some of the biggest questions
that have experts scratching their heads

110 How large is the universe?
114 how will the universe end?
120 Welcome to the multiverse
124 Space’s biggest myseries
(that we’ve solved)

110

124

© Adobe Stock; Getty; Alamy; NASA; ESA

Using advances in technology and different
methods of measurement, astronomers have
slowly increased their estimation of the size of
our solar system and the universe beyond it

Written by Nigel Watson

how large is the universe?

Our understanding of the size of our universe has changed throughout history to fit with our observations, theories and beliefs. For a large portion of human history, we thought the universe consisted of the Earth, Moon, Sun, planets and an outer layer of stars, and even 100 years ago, the Milky Way was regarded as the full extent of our universe.
At first, there was no method to determine the distance of objects beyond the Earth – but by observing when they returned to a fixed position relative to the fixed stars, it was assumed the bodies that took the longest to return were the furthest away. Thus, the Moon that only took 27 days to return to the fixed position was regarded as the closest, whereas Saturn took 29 years and was regarded as the furthest away.
Mathematician and astronomer Aristarchus Of Samos, in the third century BCE, writing in his book On The Sizes And Distances Of The Sun And Moon, claimed the Sun was 18 to 20 times more distant than the Moon, and 18 to 20 times the size of the Moon. He worked out these figures by observing that when there is a half Moon, it is at a right angle with the Earth and the Sun – so, by using trigonometry, he could work out the ratio of these distances. His calculations failed due to lack of accuracy, and we now know the average distance of the Sun from the Earth is 390 times the average distance of the Moon.
Crunching the numbers
Using the parallax method, Claudius Ptolemy (100-170) used two points on the Earth at a known distance from each other to observe the position of the Moon. As there were no accurate chronometers to precisely time the observations, they were often taken during a lunar eclipse when the ground observations could be accurately synchronised. He calculated that the Moon was at a distance of 59 times the radius of the Earth (409,000 kilometres/253,000 miles), and the Earth-to-Sun distance was 1,210 Earth radii. The sphere carrying the fixed stars was regarded as being at a distance of 20,000 times the radius of the Earth.
By the 18th century, there was an understanding of the relative distances of the planets in the solar system. However, astronomers needed a much more accurate reading of at least one planetary body that could be used as a baseline measure for calculating the distances of the rest of our solar system.
Two perfect opportunities came on 6 June 1761 and 3 June 1769 when Venus passed between the Earth and the Sun. These transits allowed astronomers based at different locations to measure the apparent location of Venus in relation to the Sun. As a transiting planet passes across the Sun, observers at the south pole would see it moving above the equator of the Sun, whereas observers on our equator would see it moving across the Sun’s equator. The parallax angle between these two observations and the precise distance between their locations enable the calculation of the absolute distance of the planetary body, in this case, Venus. The data from these observations enabled French astronomer Jérôme Lalande to calculate the distance from the Earth to the Sun as being 24,000 Earth radii.
Absolute unit
The Earth-to-Sun distance – now known as the astronomical unit (AU) – is mainly used for measuring distances in our own solar system. It was not until 2012 that the International Astronomical Union defined the astronomical unit as equal to 149,597,870.7 kilometres (92,955,807.2730 miles), 23,455 Earth radii.
When dealing with stars, Galileo Galilei (1564-1642) noted that the star Vega, with a magnitude of 0.03, appeared 360 times smaller than the Sun, so he could infer that it was 360 times further away. Whereas the faintest stars of a sixth magnitude are six times smaller than stars like Vega and are, therefore, 2,160 times further away. Now we know that sixth-magnitude stars are several hundred light years away and more than 10 million astronomical units away from us.
Calculating the stellar parallax of a star has been another method to find out how far away it is. This is obtained by observing a star’s position and measuring it six months later. Since Earth will have travelled two astronomical units between the measurements, this provides a longer baseline than just using different locations on Earth at the same time when measuring objects in the solar system. Any variation in the star’s position between the two observations can thus be used to calculate its distance.
In 1838, German astronomer Friedrich Bessel announced he had successfully used stellar parallax to measure the distance of the star 61 Cygni, in the Cygnus constellation. Although it was a faint star, Bessel selected it because it seemed close to our Sun. Using a telescope fitted with a Fraunhofer heliometer, he found the parallax of 61 Cygni was 0.314”, which gave it a distance of 10.3 light years away. We now know it is 11.4 light years.
Going the distance
Stellar parallax methods were refined and made more accurate in the 20th century with the employment of photographic plates that by the 1960s were superseded by computer technology. Satellites like the European Space Agency’s Hipparcos and NASA’s Hubble Space Telescope have used it to make far more accurate measurements of stars in our galaxy.
To determine distances beyond the ability of the stellar parallax method, Henrietta Swan Leavitt, in 1908, discovered that by measuring the absolute brightness of Cepheid variable stars between their regular pulses of maximum brightness, their distance could be determined.
Using this method in 1919, Harlow Shapley, at the Mount Wilson Observatory, determined that our Milky Way galaxy was 300,000 light years across and believed that this was an accurate determination of the size of our universe.
Edwin Hubble, who also worked at the Mount Wilson Observatory, in the 1920s, studied Cepheid variable stars inside the M31 Andromeda Nebula and worked out it was about one million light years away, far beyond the estimated size of our galaxy. When other Cepheids were observed in so-called spiral nebulae it became obvious they were actually galaxies in their own right. It was also discovered that they displayed a redshift indicating they were moving away from us, and that the further away the galaxy, the faster it moves. Using this data, Hubble estimated that the universe was 280 million light years across.
In the 1950s, radio telescopes detected powerful emissions from what were thought to be galaxies or star-like objects, and were given the name Quasars (quasi-stellar radio sources). In the following decade, astronomer Maarten Schmidt found that they travel at an extremely high velocity from us and are at the centre of galaxies in the early stages of formation. They can outshine all the stars in their galaxy making them the most luminous and distant objects in the universe. As they were seen at a distance of 15 billion light years, this gave the observable size of the universe as 30 billion light years across.
As nothing can travel faster than the speed of light we can only detect objects to a maximum distance. So the boundary of our observable universe in all directions is 47 billion light years, and being a spherical universe it has a diameter of 94 billion light years. Taking into account the expansion of the universe this increases one light year every year. From other locations in the universe, observers will see their own view of the universe that possibly will overlap with ours. Theoretically, there is no boundary limit to the size of the universe.

Cepheid variable
stars were critical in
calculating distances
in the universe.

“the boundary of
our observable
universe is
47 billion
light years”

Friedrich Bessel
used a heliometer to
calculatethe parallax
of thestar 61 Cygni.

The Most Distant Object

The galaxy HD1 is the most distant object in the observable universe. Its light has taken 13.463 billion light years to reach us, and accounting for the expansion of the universe, it is at a proper distance of 33.288 billion light years.
Using multiple telescopes, including the Subaru Telescope, VISTA Telescope, UK Infrared Telescope and the Spitzer Space Telescope, it was located in the Sextans constellation after 1,200 hours of observation time.
HD1 is an extremely bright redshift object, indicating it is moving away from us. It was formed a few hundred million years after the Big Bang, and it is probably a galaxy that is birthing hypothetical population III stars at a rate of 100 stars a year. That is ten times more than expected from similar galaxies. The extreme ultraviolet luminosity of HD1 is very suggestive of it producing population III stars, although it could be the earliest supermassive black hole ever discovered.

Logarithmic map of the
observable universe –
towards the edges, the scale
is progressively reduced.

Hubble estimated the
universe was 280 million
light years across – which was
conservative, to say the least.

© Adobe Stock; Getty; NASA; ESA; Wikipeida Commons / Harikane et al

The Big Bang created the universe
and it continues to expand and
evolve, but what is less certain is
how the universe will eventually end

The Newtonian universe was like a complicated mechanical device that worked like clockwork. It had a set of laws and it seemed likely that it would never change and would work smoothly forever. The only thing that would upset this perfect mechanism was believed to be a future divine intervention that would destroy our world and vanquish all our sins.
Since then, our view of the universe has changed and expanded much like the universe itself. It was only in the 20th century that scientists discovered the universe is far larger than just our own galaxy, and that the comfortable Newtonian laws have a more limited value in the wider context of Einstein’s theory of relativity and the science of quantum physics.
Instead of being in a steady state, the Friedmann equations by Russian physicist and mathematician Alexander Friedmann in 1922 formulated the concept that the fate of the universe depended on its density and curvature. His equations pointed to the fact that without enough matter, the universe would continually expand, and if it had more matter, gravity would cause the universe to contract.
The Friedmann equations are based on the assumption that throughout the universe, it is basically the same and is, therefore, homogeneous. Another related assumption is that the universe is isotropic, meaning that it appears the same in whichever direction you look.
Using the Friedmann equations, Edwin Hubble calculated the rate at which the universe is expanding: this value is known as the Hubble constant. The current best measurement of the Hubble constant is between 69.8 and 73.8 km/sec/Megaparsec (Mpc). One Mpc is equal to 3.26 million light years.
To explain the Hubble constant, factors such as the importance of gravity, dark matter, dark energy, Hawking radiation, the cosmic microwave background and the Higgs field have come into play. By studying these factors that influence the speed and extent of the universe’s expansion, it can be determined what its ultimate fate might be.
The Big Freeze
The Big Freeze hypothesis depends on the universe having a density less than the critical density. If that is the case, it would mean it will expand to infinity, as there would not be enough gravitational pull to stop this happening.
In the process of expanding, the stars will eventually run out of hydrogen and helium, their orbits will decay, and without them being able to go supernova to form new stars, they will all die off.
Similarly, black holes will consume the remaining stars, and they will disappear as they lose their energy. Stephen Hawking proposed that pairs of particles would be split up at the event horizon of a black hole. One of the particles would be pulled inside the black hole, whilst the other would escape its gravity and move away from it. Known as Hawking radiation, these outward particles are made up of photons, neutrinos, and other massive particles. Over an extremely long period, the radiation loss would cause black holes to lose energy and ultimately disappear.
Near the universe’s end, it would only be inhabited by photons. These are particles that decay and are massless, fundamental packets of subatomic electromagnetic energy that travel at light speed.
As the universe continues to expand, photons will be redshifted ever further until they reach infinity, and when they reach the maximum state of entropy, the universe will have no mass or energy.
Presently, this is the most favoured prediction for how the universe will end.
The Big Rip
This theory is a more dramatic version of the Big Freeze. It proposes that since the universe is composed of 68% dark energy, it could get strong enough to violently rip it apart. As the universe expands, the ‘empty space’ inhabited by dark energy increases and, in doing so, would power the expansion to accelerate in all directions.
Some of the research to support this idea was conducted by the Supernova Cosmology Project and the High-Z Supernova Search Team in 1998. They analysed Type Ia supernovas, which are produced by the explosion of white dwarfs. As their brightness is the same, they are a useful benchmark for measuring their distance. However, it was found that they were much dimmer than expected. This led to the conclusion that they were much further away and that this must be due to the accelerating rate of the universe’s expansion.
The continuous loop of dark energy producing more space with more energy will rip the very fabric of space apart. It will drive galaxies from their local neighbourhood, stars will separate inside the galaxies, and their attendant planets will move away. As this process speeds up, the very atoms and subatomic particles of the universe will get ripped apart. Every subatomic particle will move away faster than the speed of light into infinity until nothing can interact or exist in the universe. This is estimated to take approximately 22 billion years to finish off our universe.
Much of this theory relies on the fact that dark energy would get more powerful as the universe expands, yet its power could be diluted. Currently, it is thought the strength of dark energy is universally constant and does not pose enough of a threat to produce the Big Rip.
The Big Crunch
In contrast with the Big Freeze and Big Rip theories, the Big Crunch hypothesis is that instead of the universe expanding, it will fall back upon itself. This would happen if the density of the universe is more than its critical density, and so the gravitational force would be enough to restrain everything from moving out infinitely.
Stars would collapse into existing black holes, or they would become iron stars that randomly come together to form a black hole, or matter would condense to create microscopic black holes that would instantly evaporate. Eventually, all matter will disappear.
One scenario is that the Big Crunch would be like the reversal of the original Big Bang, with the universe shrinking at a steady rate at first. As the shrinking becomes faster, the cosmic microwave background (CMB), the radiation left over from the Big Bang that permeates our universe, would heat up. Galaxies would merge, causing the photons in the CMB to blueshift as their temperature rises. Stars would begin to crash into each other, and eventually, the CMB would become hotter than the stars themselves, and being unable to radiate their heat, the stars would boil and evaporate. Only minutes before the end, the CMB heat would be enough to disintegrate all the remaining atoms and their nuclei.
When the idea of dark energy came to fruition in 1998, it was a blow to the Big Crunch theory. Rather than gravity, this dark energy was found to be the primary influence on the state of the universe. Measurements made by the Chandra X-ray Observatory in 2004 showed that dark energy has a fixed value, but it increases as the universe expands. This indicates that the universe will continue expanding and that a Big Crunch is not very likely.
The Big Slurp
The universe is filled with the Higgs field, and its interaction with particles gives them mass. If it cannot lose any energy and is in a vacuum state, things remain stable. In regular physics, things tend to stay stable, but in the world of quantum mechanics, the Higgs field might spontaneously drift into a false vacuum at a certain locality, where it would change to a lower energy state.
If it does, the Higgs field will go into a phase transition. It would create low-energy bubbles that would expand at the rate of the speed of light. Interactions between protons and neutrons would change, as would the mass of electrons. Such a quantum tunnelling event, triggered by a particle moving from one state to another, could be enough to destroy stars, galaxies and eventually the universe.
It appears that the Higgs field did change at the start of our universe. The heat and energy of that period might have triggered the end of our universe, but instead, these conditions helped stabilise and modify its quantum properties.
Research using Large Hadron Collider is helping to understand the Higgs boson particle that is part of the Higgs field. This research should find out more about its role in the beginning of our universe and how it might shape its end. Fortunately, the expert opinion is that this might not happen for a massively long time.
Predicting the End
How the end of the universe will occur is still hotly debated, and with digital technology, supercomputers and a combination of ground-based observatories and space telescopes, new information is being constantly collected and analysed.
In July 2023, the European Space Agency launched the Euclid space telescope to conduct a cosmological survey of the extragalactic sky outside the Milky Way. The telescope incorporates a near-infrared spectrometer, a photometer and a 600-megapixel camera to measure the relationship of the distance and redshift of galaxies. This mission will collect a vast database to study the nature of dark matter and dark energy and how it affects the acceleration of the expanding universe.
Euclid has already catalogued 11 million objects in visible light and 5 million more in infrared light in an area of 15,000 square degrees (a third of the sky). This data will be used to direct a deeper and more precise exploration area of 2,000 square degrees (one-twentieth of the sky) using NASA’s Nancy Grace Roman Space Telescope, scheduled for launch in 2027.
It will conduct a High Latitude Spectroscopic Survey to accurately measure the position and distances of millions of galaxies. This will help us calculate the role of dark energy since the Big Bang that has influenced their distances and distributions.
The High Latitude Imaging Survey will measure the degree of distortion of space-time caused by gravity in galaxy clusters, and this will help calculate the mass and influence of visible stars and dark matter. By these means, we should get a better picture of how dark energy has affected the structure of the universe.
A third aspect of this mission will be to go back to studying Ia Supernova, which originally helped prove that the universe is expanding – and at a rate that seems to be accelerating. The telescope will determine the distance of these stellar objects by their brightness, and the wavelengths of light emitted by these dying stars will be studied to calculate the speed they are moving away from us.
The results from these three survey methods by the Nancy Grace Roman Space Telescope and collaboration with the Euclid telescope data will give us a far better understanding of the universe’s expansion since the Big Bang. This work will also be able to judge the nature and role of dark energy or whether we need to change our idea of how gravity functions. Either way, we should get a better idea about the fate of our cosmos.
Whether it will be the Big Freeze, the Big Rip, the Big Crunch, the Big Slurp or something equally ‘Big’, we can be assured that any of these events will happen a long time from now, so we don’t need to worry about the universe ending anytime soon.

Some theorise that the
exponential growth of
dark energy could rip
our universe apart.

© Adobe Stock; Alamy; Getty

In many scenarios,
black holes are the
objects that will
linger the longest.

Another universe-ending
concept has it collapsing
in on itself, like a
reverse Big Bang.

Infinite universe expansion
could cause a heat death
as everything drifts further
and further apart.

how will the universe end?

“when photons
reach the
maximum state
of entropy, the
universe will
have no mass
or energy”

Signs

of the end

Death of our Sun
In 5 billion years, our Sun will use up its hydrogen fuel. The outer layers will drift into space, and the remaining core will become a dense, shrunken, cool, white dwarf star.
End of Star Formation
In about 100 trillion years’ time, all the stars will run out of hydrogen and will stop forming. Low-mass stars will become white dwarfs, whilst high-mass stars will become black holes or neutron stars.
Decay of Protons
When the universe is at a temperature near absolute zero in 10 trillion trillion trillion years, positrons will be produced by decaying protons. On encountering electrons, the positrons will be annihilated to create gamma ray photons.
Black Holes Disappear
In 10 thousand trillion trillion trillion trillion trillion trillion trillion trillion years, the only remaining celestial objects will be black holes that will emit Hawking radiation, causing them to lose mass until they go into oblivion.
Low Energy State
With the demise of protons and black holes, only neutrinos, electrons, positrons and photons of enormous wavelengths will exist in 10150 years’ time.

Return of the Universe

The Big Bang, expansion and eventual death of the universe might not be a one-off event. Instead, the death of the universe could trigger a new beginning when there is another Big Bang and another cycle of existence and death.
A team of international scientists proposed that primordial black holes (PBH) are the remnants of a previous universe, and this provides an explanation for the existence of dark matter and why the universe appears to be flat.
This helps support the concept of Conformal Cyclic Cosmology put forward by physicist Roger Penrose, whereby the universe is completely consumed by black holes and no longer appears to have a physical presence. It would continue forever, and from a different timeline, it would give rise to a new universe. Penrose called each of these cycles ‘aeons’, where each mainly follows the same life cycle and physical laws of the previous aeon.

The Large Hadron Collider
at CERN is instrumental in
helping us understand the
Higgs bosun and Higgs field.

Equipment like the
Nancy Grace Roman
Space Telescope will further
our understanding of the universe.

More research into
type Ia supernovae
will help us understand
universe expansion.

The ESA’s Euclid space
telescope will study
the extragalactic sky
outside the Milky Way.

© Adobe Stock; Getty; NASA; CERN

The concept of multiverses beyond or within
our universe has inspired all manner of
fanciful ideas and philosophical discussion,
but is it worthy of scientific study?

Welcome to the Multiverse

The idea that there are other universes, so-called multiverses, beyond our own has been a feature of science fiction since the 20th century. Recently, the 2022 Oscar-winning film Everything Everywhere All At Once popularised the concept, and as far back as 1937, Olaf Stapledon, in his Star Maker novel, eloquently describes this idea: “In one inconceivably complex cosmos, whenever a creature was faced with several possible courses of action, it took them all, thereby creating many distinct temporal dimensions and distinct histories of the cosmos. Since in every evolutionary sequence of the cosmos, there were very many creatures, and each was constantly faced with many possible courses, and the combinations of all their courses were innumerable, an infinity of distinct universes exfoliated from every moment of every temporal sequence in this cosmos.”
Opening our minds
Supporters of this theory argue that the idea of this being the only universe is as comparable and as narrow-minded as earlier thinking that our planet was at the centre of the universe. The anthropic principle is that our universe is finely tuned and has all the parameters for our existence. This has been criticised as being a form of ‘carbon chauvinism’ that assumes life is necessarily carbon-based and does not take into account other chemical elements, such as silicon, that might also form complex structures to create different life forms.
Philosophers and theologians have debated the possibility of multiverses, often in terms of the nature of individuality and free will, and what do we do with the knowledge that our actions could change other universes? In the religious context, it challenges whether a higher power is the creator of just this universe or of all the multiverses.
These hypothetical musings have challenged our understanding of the cosmos, but are they fantasies and philosophical debating points rather than something science can study? Given the complexity of our known universe, scientific theories about multiverses have come increasingly to the fore.
Hugh Everett III was one of the earliest to seriously consider the scientific possibility of other multiverses. He pondered over the question of why, in our normal reality, we observe and experience things in a linear, stable fashion, but in the microscopic world of quantum mechanics, particles, like electrons, are not in a single state but can appear in different locations, and at different velocities and orientations simultaneously. This led him to wonder if the universe is full of multiverses that, like electrons, can exist simultaneously. These quantum multiverses would constantly split, and decisions in our own universe could influence their existence and create different versions of ourselves.
Everett’s 1957 Princeton doctoral dissertation, The Theory Of The Universal Wavefunction, outlined these ideas. The wave function of the universe describes the interactions of matter and energy throughout the universe as a quantum system. This concept became more widely known when theoretical physicist Bryce DeWitt wrote about them in Physics Today in 1970. It was DeWitt who termed this the many-worlds interpretation (MWI).
The unbuildable machine
David Deutsch, a quantum physicist associated with the University Of Oxford, was impressed by Everett’s MWI. He also supported the idea that every decision and choice we make in this world will create other universes where the alternative choices are acted out by our duplicate selves. In 2022, he was awarded the Breakthrough Prize in fundamental physics for his paper written in 1985 that proposed building a rudimentary quantum computer that would test the existence of other universes. Although this exotic machine is currently unbuildable, the concept of a computer with quantum components earned him the title of “father of quantum computing”.
Russian physicist Andrei Linde, at Stanford University in California, put forward his ‘eternal chaotic inflation’ theory in 1986. Rather than the universe expanding like a single, vast balloon from the Big Bang, Linde proposed that our universe is just one of many universes. This would have happened within a fraction of a second after the Big Bang. In that brief moment, quantum fluctuations could have created these other ‘bubble’ universes that would have expanded and become entities in their own right. Each universe would have different laws of physics, and the chain of creation of universes would be unending and completely separate from our own.
A similar concept is that of braneworlds that exist in different parallel dimensions. They are bunched together, and if they collide, it is possible that the result would be to create a Big Bang, thereby creating another braneworld. In our own braneworld, we are aware of the three dimensions of space and the fourth dimension of time, but we cannot detect other braneworlds because they exist in completely different dimensions.
Max Tegmark, a professor at the Massachusetts Institute Of Technology, has put forward his mathematical universe hypothesis (MUH). This hypothesis consists of four levels, starting with a universe that is an extension of our own, universes with different effective physical constants, dimensionality and particle content, the MWI and fourthly, an infinite variety of mathematical structures.
In his book, The Hidden Reality: Parallel Universes And The Deep Laws Of The Cosmos, American physicist Brian Greene argued that there are 11 types of universes. This includes the landscape multiverse, which states that different shapes for string theory’s extra dimensions give rise to many different bubble universes. In an inflationary multiverse, its continuing expansion features a vast network of bubble universes, including our own. The brane multiverse exists on different dimensional levels. In an infinite universe, a quilted multiverse could be created that repeats every event across space an infinite number of times. Or we might be in a simulation or a holographic multiverse.
Clues in the CMB
Rather than just speculation, there is some evidence suggesting the existence of multiverses. In 2010, a multinational team found four unusual circular patterns in the cosmic microwave background (CMB) that they attributed to being caused by other universes bumping into our own.
In 2017, Professor Tom Shanks at Durham University’s Centre For Extragalactic Astronomy – using spectrographic data from the Anglo-Australian Telescope in New South Wales – found a cold spot in the CMB. It did not conform to any standard universe model, making Shanks conclude it was caused by a collision with a bubble universe and the possibility that billions of other similar universes may exist.
Researcher Ranga Ram Chary at the European Space Agency (ESA) also studied the CMB using data from the Planck space observatory and found extremely bright areas at a specific frequency. As they were unrelated to visible stellar objects, he suggested they indicated where our universe has impinged upon other parallel universes.
As might be expected, alternative and less exotic explanations have been put forward to account for those findings. Equally, the MWI and other multiverse theories have been criticised. For example, theoretical physicists Professor Sandu Popescu and Dr Daniel Collins at Bristol University in April 2024 published their paper Conservation Laws For Every Quantum Measurement Outcome, which states that the laws of the conservation of energy are obeyed throughout the universe and do not need a MWI to explain them.
It has been argued that multiverses are separate entities at vast distances that would never interact with our universe. It might be possible to detect them or their signature, but it is more likely they are so different that we are unable to perceive or understand them as they are beyond our own time/space/dimension.
Stephen Hawking was a critic of multiverse theories, stating, “The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite, the theory can’t be tested.”

Planck’s observations
of the CMB could
have revealed clues of
multiple universes.

The film Everything
Everywhere All At Once
deals with multiverses in
a highly entertaining way.

Existence in

Another universe

It is very likely that the laws of physics could be very different in another universe, and what we regard as normal could be completely changed.
Our own universe seems fine-tuned for our existence, with no sign of any other intelligent life forms. In other universes, conditions for humans to exist could be impossible due to their temperature and structure, the manner of their evolution, a change in the force of gravity and time might even run backwards. These factors could produce strange forms of biological and sentient beings that thrive in these different universes, or life forms might not evolve at all.
Alternatively, multiverses are variants of our own universe with the same laws but with an infinite variety of ourselves living out different lives with differing outcomes. If you ever find yourself curious about what could have happened if history panned out differently, keep an eye out for our What If sister bookazine series.

It’s theorised that
branching decisions
we make spawn and
influence other worlds.

Some speculate
multiple universes
were created with
the Big Bang.

Astronomers and
scientists, over hundreds
of years, have
discovered the extent
of our universe and the
laws that govern it

Heliocentric Model

The geocentric model of our universe was first put forward by the Ancient Greeks, and it supported the belief that if a higher power created humanity, then it followed that it would put us at the centre of creation.
The model was first conceived by Eudoxus in 380 BCE as a series of 27 concentric spheres moving around a stationary Earth and was refined by Claudius Ptolemy to account for the eccentricity of planetary motions. This concept dominated until Nicolaus Copernicus, writing in the early years of the 16th century, argued that the Earth spins on its own axis and that, along with the other planets, circles the Sun.
The so-called Copernican Revolution that established the heliocentric model over the geocentric model had several significant milestones. Danish nobleman Tycho Brahe’s observations using a compass and sextant, for example, improved the measurements of astronomical movements, making them valuable to support the new hypothesis. His observations led him to conclude that the five planets circled the Sun and were not attached to any solid spheres, yet he still thought the Sun, the stars and the Moon orbited the Earth. He published details of his halfway house geoheliocentric concept, known as the Tychonic system, in 1588.
Using data gathered by Brahe, German astronomer Johannes Kepler and his own ten-year-long study of Mars was able to further establish the validity of the heliocentric model by understanding the motion of the planets.
The invention and use of telescopes, especially by Galileo Galilei, further drove a nail into the geocentric model. It was in January 1610 that he observed four ‘stars’ around Jupiter that, over a few nights, changed position, indicating that they were orbiting this planetary body, something not possible in the geocentric model.
A combination of mathematics and actual observations eventually made the geocentric model scientifically untenable.

Our views and understandings of the universe have changed dramatically throughout history. From the beginning, mythological events and beings were an essential component of creation stories, and even the scientifically inclined Ancient Greeks regarded the universe as consisting of concentric spheres that surrounded the Earth.
It was during the Renaissance in Europe that the invention and use of telescopes advanced the view that the Sun, rather than the Earth, was at the centre of the universe. Galileo Galilei was one of the first to build and use a telescope for astronomical purposes, and he refined the instrument to magnify by 30 times. His rapid publication of his observations, especially of the four moons orbiting Jupiter, helped support the heliocentric concept of our solar system.
This underlines that technological advances, from telescopes and spectrographs to radio astronomy and space telescopes, have progressively given us vast amounts of information that have changed long-held beliefs and theories.
As science discovers new things that help solve old enigmas, it also reveals new mysteries to solve. Isaac Newton’s concept of Universal Gravitation was a significant theory of a universe that worked with clockwork precision. Yet, it had irregularities and contradictions that Albert Einstein sought to resolve in his Theory Of Relativity.
Advances in science and the ability to see further into space – using unmanned space telescopes – have helped support the Theory Of Relativity. Still, it is in contradiction to quantum mechanics, which deals with the behaviour of subatomic particles. So, there are plenty more cosmological mysteries to solve or at least resolve.

The Ptolemaic
geocentric model
drawn by cosmographer
Bartolomeu Velho.

© Adobe Stock; Alamy

Planetary

Motion

It was very difficult to explain the motion of the planets – if they were not attached to spheres, what caused them to move? In 1609, Johannes Kepler published his Astronomia Nova (New Astronomy), where he described two laws of planetary motion, and ten years later, he added a third law.
Previously, the heliocentric model proposed by Copernicus regarded the planets as moving in perfect circular orbits and epicycles. Instead, Kepler determined that they actually move in elliptical orbits.
Firstly, The Law Of Ellipses states that the planets move around the Sun in an elliptical orbit and that the Sun is at the centre of one of the two foci of the ellipse.
The second Law Of Equal Areas takes into account that a planet travels faster over a larger area when it gets closer to the Sun and slower over a smaller area as it moves away from the Sun.
Having described the motions of single planets, Kepler’s Law Of Harmonies notes that the ratio of the squares of the orbital period of any two planets in the solar system is equal to the ratio of the cubes of their average distances from the Sun. Using this calculation, it was found that all the planets have a very similar orbital period. This is the period that it takes for one object to complete an orbit of another object – in the case of the Earth, it takes 365 days/one year to orbit the Sun, and its average distance from the Sun is one astronomical unit. Saturn, the farthest known planet at that time has an orbital period of 29.4 years around the Sun and an average distance from the Sun of 9.5 astronomical units. This law takes into account that the orbital period of a planet is longer the further away it is from the Sun.

An excerpt of Kepler’s
Astronomia Nova,
which describes his laws
of planetary motion.

“Kepler determined Planets move in elliptical orbits”

Johannes Kepler
(1571-1630) advanced
our knowledge of
planetary movement.

Kepler’s three laws
of planetary motion
helped lay the ground
for our understanding
of the solar system.

Galileo threw weights
from the Leaning Tower
Of Pisa, kickstarting an
understanding of gravity.

Universal

Gravitation

The problem for Kepler and astronomers of that time was determining what caused celestial bodies to move around the Sun. One idea was that the Sun had a magnetic force that moved the planets.
A clue to the role of gravity was provided by Galileo, who demonstrated, by throwing different weights from the top of the Leaning Tower Of Pisa, that heavy and light objects – regardless of their shape, size or mass – fall back to Earth at the same rate.
Using Kepler’s third law of planetary motion and Galileo’s findings, Isaac Newton formulated the concept of Universal Gravitation. This was able to explain why planets moved in an elliptical orbit rather than just flying away into outer space
Newton determined the force that causes an object to fall to Earth – for example, the apple that allegedly fell on his head – is universal and can be applied to the planets. He argued that if you fired a cannonball horizontally from a mountain, the force of gravity would eventually bring it back to Earth. Firing the cannonball at a much faster speed would cause it to fall in a circular orbit around the Earth, much in the way the Moon does. If fired at an even greater speed, the cannonball would enter into an elliptical orbit of the Earth, much in the way the planets orbit the Sun.
In mathematical terms, Newton proved that this force of attraction between two objects was determined by their proportional masses and inversely to the distance between their centres. Gravitational attraction is stronger when two large masses are close together, and if they move away from each other, the gravitational attraction weakens.
In the case of our solar system, the gravitational force of the Sun keeps the planets in their elliptical orbits, and the gravitational force of the planets keeps their moons from flying away from their orbits.

Theory

of

Relativity

Isaac Newton’s theory of gravity, published in Principia dated 1687, held sway until 1905 when Albert Einstein published his Theory Of Special Relativity. Einstein pointed out how observers perceive events differently depending on their relative motion and that the speed of light is constant.
One illustration of special relativity is that if you are in a stationary position, you will measure light moving past you at a speed of 186,000 miles per second. However, if you are moving at a speed of 10,000 miles per second, you will record the speed of light moving past you as 176,000 miles per second. Both observations are correct, and such concepts led to Einstein postulating that time and space are intimately linked.
Ten years later, he published his Theory Of General Relativity, which dealt with the nature of gravity. He proposed that massive objects warp space-time, and this force manifests itself as gravity. One example is when light passes a star, it will be deflected to follow the curvature of space-time.
For example, according to Newtonian principles, Mercury should follow a different direction of 5,600 arcseconds per century as it gets closer to the Sun. There is a discrepancy of 43 arcseconds per century, and this is only accounted for by Einstein’s concept of curved space-time. Such a minute wobble, where one arcsecond equals 1/3600 of a degree, will – over a few billion years – cause it to crash into the Sun.
Einstein’s theory also explains that due to gravitational time dilation, processes near a massive body run more slowly and that violent events like the collision of black holes can cause small ripples in time and space that he termed gravitational waves.

According to Einstein’s
theory, large bodies
like planets and stars
warp space-time.

Einstein’s theories and
intellect made him
famous outside the
world of science.

Big bang

It is one thing observing and attempting to explain how the universe works, but it is another matter altogether when we try to explain how the universe came into existence.
Early cultures proposed mythological origins for our universe. The Ancient Egyptians believed that from dark, chaotic primordial waters, the god Ra was born, and he instituted order and light to the world. Other cultures, such as Chinese, Greek, Roman and Hindu mythology, believed that a cosmic egg was instrumental in bringing forth our world either by fragmenting itself into elements of the universe or by birthing deities.
The Christian Book Of Genesis tells us that God brought about the universe six days after commanding, “Let there be light.” Some believe this was literally true, whilst others interpret it as being a symbolic way of conveying that there is only one God and we are created in his image.
In the scientific world, there was support for a Steady State theory of the universe put forward by Fred Hoyle, Hermann Bondi and Thomas Gold in 1948. This regarded the universe as infinitely old, infinite in size and that, on a large scale, it does not change. On a smaller scale, it acknowledged that stars did burn up to become neutron stars, black dwarfs and black holes, but they were continually replaced by new stars.
Fred Hoyle derided what he coined the Big Bang theory as he thought the influence of the Book Of Genesis overshadowed the thinking of scientists. The Big Bang theory explains that about 13.8 billion years ago, the whole of space, time, matter and energy emerged from the minute and intensely hot singularity and has since expanded and cooled to form stars and galaxies.
The discovery of quasars, the age of stars, the motion of galaxies and other observations in recent years have supported the Big Bang theory and it offers the best explanation for our creation.

“Early cultures
proposed
mythological
origins for our
universe”

The galaxy NGC 4603 was
subject to a Hubble Space
Telescope study to locate
Cepheid variable stars.

By observing other
galaxies, Hubble was
able to grant us a wider
picture of the universe.

Edwin Hubble’s work
was so significant he
had a space telescope
named after him
posthumously.

Age

of the

Johannes Kepler and Isaac Newton regarded passages in the Bible as historically correct, and by studying them, they concluded the Earth was 6,000 years old.
By calculating the chemical reactions needed to keep the Sun burning, astronomers at the end of the 19th century thought that it was about 100 million years old. In the 20th century, the study of radioactive decay in rocks and asteroids found that they were billions of years old. The new science of nuclear physics enabled a better understanding of the interior nuclear processes inside our Sun, and this found it was 4.5 billion years old. This evidence leads to this figure being the same for the Earth and the solar system.
A great step in discovering the age of the universe was taken by the work of Edwin Hubble in the 1920s. By studying Cepheid variable stars located in spiral nebulae, he found that they must exist far beyond our Milky Way galaxy. Hubble was able to announce these nebulae are actually galaxies in their own right, proving that our Milky Way is just a diminutive part of the universe.
Another discovery was that the spectra of the light emitted by galaxies shifted towards the blue end of the spectrum when moving towards the Earth and that those moving away from the Earth showed a shift to the red end of the spectrum.
From studying 18 galaxies, Hubble determined that the further away the galaxy is, the faster it is moving away from us, which became known as Hubble’s Law. From his measurements, he reasoned that the universe began in a single burst of energy and has been moving away from this point of origin ever since. Hubble’s groundbreaking work led to the support of the Big Bang theory and ageing the universe at 13.7/13.8 billion years.

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COSMIC

Foundations

Unravel the secrets

of the Big Bang

Solar system

The unexplored

reaches of our home

Alien worlds

Will we ever contact

extraterrestrials?

Galactic

Enigmas

The great attractor

quasars and more!