In fact, polls in Australia, the US and elsewhere indicate around a third of the public believes aliens are here.

However, while what we know about the universe suggests aliens may exist, there are three compelling reasons why they probably aren’t visiting us.

Space is big – very big

To begin with, space is vast – beyond our imagination.

Proxima Centauri, the nearest star to our Sun, is about 40 trillion kilometres away, 268,000 times farther than the Sun is from Earth. That’s 4.3 light years as astronomers measure it. A light year is the distance light travels in one year at 300,000km per second.

We can only travel across space at a fraction of the speed of light with current technology. Even our fastest spacecraft, the Parker Solar Probe, travels at a top speed of roughly 191 kilometres per second – 0.064% the speed of light.

At that speed, it would take about 6,650 years to reach Proxima Centauri, and that’s just in our local stellar neighbourhood. So interstellar travel within human lifespans would require much higher velocities.

Let’s assume we did have the means to travel close to the speed of light. That introduces the first problem with travelling at that velocity. Albert Einstein demonstrated that time is relative; the rate of time flow is not the same everywhere in the universe. The faster a spaceship travels from Earth, the slower time will pass for its passengers. This is called time dilation.

For example, when NASA astronaut Scott Kelly arrived back on Earth from a year on the International Space Station, he was milliseconds younger than his identical twin because time moves more slowly for objects in motion, and the International Space Station travels at roughly 28,150 kilometres per hour.

This difference was negligible for the Kelly twins. But for any aliens cartwheeling through our skies, it would be significantly more because of the journey to Earth and back from a distant star system at a necessarily higher speed. They would go home to a planet much older than the one they left – perhaps by a century or more. They would be time exiles.

Unimaginably high energy requirements

Then there’s the unimaginably high energy requirement for interstellar travel.

The mass of the spaceship increases with velocity, so an increasing amount of energy is required to accelerate it.

At the speed of light, the ship becomes infinitely massive, requiring an infinite amount of energy. This is clearly impossible.

Another significant issue is that space is a vacuum – but not completely. There are just enough particles to worry about. They can potentially cause fatal radiation for passengers and the instruments of a high velocity spacecraft, or destroy it. Sparsely spread hydrogen atoms turn into intense radiation at near light speed, and the heat that is generated would ablate and eventually destroy the hull.

Faster-than-light travel, according to physicist Miguel Alcubierre, is possible, but it comes with its own set of issues and a currently impossible energy requirement.

That raises the question of why spend all this energy to travel to Earth? Anything we have, an advanced civilisation (as they would have to be to get here) would be able to make on their planet.

A unique biosphere

Yet another issue is our biosphere, unique to Earth as far as scientists know.

Life and the planet co-evolved. Complex life would not exist on Earth if cyanobacteria, a type of single-celled microbe, had not pumped oxygen into our mostly nitrogen atmosphere 2.4 billion years ago.

It’s therefore not toxic for us, but oxygen is reactive and could be highly corrosive for aliens. And while they could wear protective suits like humans do when going to inhospitable environments, reports of visiting aliens do not include any descriptions of spacesuits.

So, are aliens out there?

If aliens are not here, are they out there?

It’s an interesting question, scientifically and philosophically. Scientists do not have enough information yet, but they are working on the question.

About 6,200 exoplanets have been found in more than 4,700 solar systems, though none are like Earth or our Solar System.

Most stars could have at least one planet, and there are more than 100 billion stars in our galaxy alone. The number of planets is therefore astronomical, and some may be habitable.

Closer to home, there are worlds with potential for microbial life either past or present – Mars, Europa (a moon of Jupiter), and Enceladus and Titan (moons of Saturn). If we discover life began twice in our Solar System, that will increase the odds of life elsewhere.

Since 1960, we’ve had the capability to look for intelligence elsewhere, piggybacking on normal radio astronomy. The biggest search for alien life projects are carried out by the SETI Institute in California and the Breakthrough Listen project based at Oxford University in the United Kingdom.

Nothing has been found across all the searches made. Finding intelligence in our time frame – about a hundred years – in the 13.8-billion-year history of the universe is challenging.

However, as a 1959 Nature paper noted, while it’s difficult to estimate the chance of success, if we don’t search, the chance drops to zero.

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The speed of the fireball is one indicator of this – the faster an object is moving when it hits Earth’s atmosphere, the more different its orbit around the Sun must be to that of Earth. The high speed of this fireball’s entry suggests it was likely moving on quite an eccentric orbit around the Sun.

In technical terms, the fireball was a “bolide”. Bolides are meteors that are not only brighter than the planet Venus, but which can also seen to explode or break up as they enter the atmosphere. They are rare to see.

More importantly, however, this is all part of disentangling the heritage and history of the Solar System’s formation. Every object that enters our atmosphere in this way is a pristine piece of material that can add to our story of the Solar System’s history, and from which we can learn about its current state.

Another hint comes from the bright explosion of the fireball. That means it was to some degree a fragile object – icy or rocky. By comparison, a solid lump of iron from the core of a shattered larger asteroid would be strong enough to plough through Earth’s atmosphere without fragmenting, making such a terminal explosion much less likely.

First, what it wasn’t

The only way to know for certain what the fireball was made of is to find any pieces of it that reached ground level, and analyse their chemistry in the lab. But given the bright terminal explosion, it’s unlikely any solid material survived the blazing atmospheric entry.

But why might scientists want to know this?

The first point to make is that this fireball was not a piece of space junk. It was moving very fast, likely in excess of 30 kilometres per second.

For one thing, it’s interesting to get an idea of the fireball’s provenance. Was it originally from the asteroid belt? A fragment of a comet? Can we link it to a known object?

A fragile fragment

But that does not mean we will never learn the truth of the fireball’s origin. Scientists will gather as much footage of the fireball as they can. This will allow them to triangulate and precisely retrace the fireball’s path, and to accurately determine how fast it was moving and at what altitude it exploded. It will even reveal details on the object’s orbit around the Sun, prior to encountering Earth.

These are all crucial clues in this astronomical detective story.

It is this superheated air that provides the vast majority of light from a meteor or fireball, and therefore what creates the striking colours. Many bright fireballs are seen to have a greenish hue, which is often stated as evidence for an iron/nickel composition. In fact, that greenish glow is so commonly observed because it is the result of superheated atmospheric oxygen.

In an ideal world, scientists would be able to get their hands on pieces of this new interloper – fragments of material dating back to the Solar System’s birth. But even by simply gathering information on the object’s orbit prior to encountering Earth, we can learn a great deal.

So where might this fireball have come from?

Stunned onlookers from Sydney to Canberra, and beyond, reported seeing the explosion light up the night sky in colourful streaks ranging from blue to green to orange.

Space junk, in contrast, enters Earth’s atmosphere at slower speeds of roughly 8km per second. In addition, such junk enters Earth’s atmosphere at a very shallow angle, meaning it can streak from horizon to horizon over the course of a minute or more.

At about 6:30pm last night, a meteor exploded with a bright flash that was widely seen across eastern Australia.

That’s because the vast majority of the observed colour from a fireball is associated with the gases in Earth’s atmosphere. As the fireball passes through the atmosphere, it causes a massive shockwave. This causes the air in front of it to rapidly heat up.

Another important point is that the colours observed as the fireball flew through the atmosphere and exploded are not necessarily strong indicators of exactly what it was made of.

Unravelling the Solar System’s history

Also, the direction of the observed fireball suggests it entered Earth’s atmosphere above the ocean. So if there are surviving pieces, they are probably buried at sea.

The second point to make is that this fireball was definitely not from the Eta Aquariid meteor shower, which is visible between mid-April and late May.
Meteor showers consist of debris moving through space, crashing into Earth, and coming from a specific direction. Meteors in a shower appear to radiate from a single point in the sky, known as the radiant, which gives the shower its name.

It’s more likely this fireball was an icy fragment of a comet, or a rocky fragment of an asteroid, from the outer reaches of the Solar System.

Crucially, if a meteor shower’s radiant is below the horizon, you cannot see meteors from that shower. Earth is in the way, and the debris is hitting the other side of our planet! The radiant for the Eta Aquariids rises during the early hours of the morning across Australia. So a fireball seen in the evening sky cannot be related to the shower.

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In the local parts of our galaxy alone there’s a complex of roughly 15 individual interstellar clouds. The Solar System is currently traversing one of them, aptly named the Local Interstellar Cloud. The origin and history of these clouds are believed to be tightly connected to the birth and death of stars. But we can see their imprints right here on Earth, in a place you might not expect – Antarctic ice.

My colleagues and I have been studying stardust trapped in old Antarctic snow and ice to trace the history of our solar neighbourhood, including the Solar System itself.

In a new study published in Physical Review Letters, we found a subtle clue that reveals our Solar System’s movement through the local interstellar environment over the past 80,000 years.

Looking down to see the sky

Astronomy usually looks outward. Telescopes collect light from distant stars and galaxies, allowing us to observe events across vast stretches of space and time. From these observations, we infer how stars live and die, how elements are formed, and how the universe evolves.

Our approach turns that idea on its head.

Instead of observing the light coming to us, we study the debris of exploding stars right here on Earth. As cosmic furnaces, stars forge many elements in their cores, from carbon and oxygen to calcium and iron. This includes rare isotopes (variants of chemical elements) such as iron-60.

When massive stars explode into supernovae at the end of their life, these elements are ejected into space and become interstellar dust.

Tiny grains of this dust then drift through the galaxy and occasionally find their way to Earth’s surface. Radioactive iron-60, a fingerprint of stellar explosions, is embedded within these grains. By searching for these atoms in geological archives on Earth, we can probe astrophysical events like supernovae long after their light has faded.

This is why Antarctica is so valuable. Its snow accumulates slowly and remains largely undisturbed, forming a layered record that stretches back tens of thousands of years. Each layer captures a snapshot of the material that was present in our cosmic neighbourhood at the time.

Finding stardust in Antarctic ice

When we studied 500kg of recent snow in Antarctica, we unexpectedly found this rare radioactive isotope. Where did it come from? There was no recent near-Earth supernova.

But our solar neighbourhood is filled with 15 clouds, with the Solar System currently traversing at least one of them. Is the stardust waiting in the clouds to be picked up by Earth? If yes, then the amount of stardust Earth collects should be related to their structure: the denser the clouds, the more iron-60 they contain. This was our educated guess in 2019.

Soon, other explanations were brought forward. Millions of years ago Earth received large showers of iron-60 from massive supernovae. Is the iron-60 in Antarctic snow the last remnant or an echo of this signal? A rain that became a drizzle?

To find out, we analysed a 300kg section of Antarctic ice, dating from 40,000 to 80,000 years ago. The process is painstaking. The ice needs to be melted and chemically treated to isolate tiny amounts of iron, including the iron-60 from the stardust.

Then, using the sensitive atom counting technique of accelerator mass spectrometry at the Heavy-Ion Accelerator Facility at Australian National University, we counted individual atoms of iron-60.

The expectation was straightforward: based on previous measurements from surface snow of Antarctica and several thousand-year-old ocean sediments, we anticipated a certain steady level of iron-60 deposition.

Instead, we found less. Not zero, but noticeably lower than expected.

This result suggests that less interstellar dust was reaching Earth during that period. This is a remarkable change on a comparatively short astrophysical timescale and does not fit the long timescales of the iron-60 deposits that landed here millions of years ago. Instead, we needed to look for a smaller, more local source for the isotope.

A fitting story

Naturally, astronomers are also quite interested in the clouds around the Solar System. Last year, a study reconstructing the history of the clouds arrived at the conclusion that they most likely originated in a stellar explosion. Furthermore, they found the Solar System has been traversing the Local Interstellar Cloud from sometime between 40,000 and 124,000 years ago.

If that’s correct, we would expect that the amount of iron-60 collected on Earth should have changed sometime in the same time period – between 40,000 and 124,000 years ago.

This is exactly what our results showed in Antarctica.

The story doesn’t fit perfectly, though. If these clouds did originate directly from an exploding star, we would expect way more iron-60 than we actually see in Antarctic ice.

Nevertheless, these clouds are imprinted in Earth’s geological record. If we look deeper and analyse even older ice, we might soon unravel the mystery of these local interstellar clouds, revealing their full history and uncertain origins.

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