Whether it’s giant war-mongering spaceships, green humanoid Martians, or creepy beings slithering across uncharted planets, science fiction has painted a very particular image when it comes to aliens.
But while you might think it takes a tin-foil-wearing conspiracy theorist to believe in such life outside of Earth, many highly distinguished scientists are believers too.
In fact, when surveyed, 86.6 per cent of astrobiologists recently said they either agreed or strongly agreed that extraterrestrial life likely exists somewhere else in the Universe.
When asked the same question but for complex or intelligent life, the agreeing percentage only drops to 58.2 per cent. More believe than don’t.
So, what do these scientists know that we don’t know? And why are they so sure we’re not alone in this Universe?
Why aliens just might exist
There is nothing special or remotely unique about Earth or humanity. That’s not our opinion – that’s the overarching idea of the Copernican principle. This is the belief that, while we often see Earth as superior, it is no more special than any other planet.
But what does that have to do with aliens? Well, many who study extraterrestrial life believe that Earth’s habitable state, combined with the development of diverse intelligent life, is a sign that the same is possible elsewhere. And given the sheer scale of the cosmos, some scientists argue the question isn’t if aliens exist, but where.
For instance, our galaxy contains somewhere between 100 and 400 billion stars. Astronomers have already confirmed around 4,000 planets orbiting other stars – and that’s in our galaxy alone. The Milky Way itself is just a tiny fraction of the observable Universe, about one millionth of it.
Scale that up, and you’re looking at roughly 100 sextillion stars (100,000,000,000,000,000,000,000) – each potentially hosting planets.
That’s an incomprehensibly large number. For instance, it’s not that there's likely more planets in the Universe than grains of sand on Earth, but 13.33 billion times more. With odds like that, it feels almost inevitable that at least one of them has given rise to intelligent alien life.
“We now know that most stars have a planetary system. That means there are loads of places where life can arise. Besides, simple life arose so quickly on Earth. To me, that’s a key sign that it must be happening elsewhere,” says Mike Garrett, chair of astrophysics at the University of Manchester.
And it’s not just the sheer number of planets that makes alien life seem plausible – it’s what they’re made of. The basic ingredients for life are more common than you might think.
Carbon, hydrogen, oxygen and nitrogen – the elements that make up living things on Earth – have all been detected on comets, asteroids and distant worlds. Astronomers have also identified many planets where water, another key ingredient, is present.
Take KOI-5715.01 – an exoplanet around 3,000 light-years from us. The name might not sound catchy, but this world is one of the most Earth-like we’ve found so far.
It’s a similar size to our planet, sits in a habitable temperature range, and shows signs of water – making it a prime candidate in the hunt for alien life.
We can also turn to Earth as a sign that life can thrive in places once thought impossible. From acidic hot springs to deep-sea vents and the frozen Antarctic, a host of recently discovered 'extremophiles' show that living organisms can adapt to far harsher conditions than a mild, Earth-like planet.
Giant tube worms, for example, thrive in the pitch-black depths of the ocean, clustered around hydrothermal vents that spew out scalding, mineral-rich water at temperatures of up to 350 °C.
Despite the intense heat, crushing pressure and complete absence of sunlight, these worms form vast colonies, relying on symbiotic bacteria to turn the vent’s chemicals into energy.
Even more extreme is Deinococcus radiodurans, a microbe whose name roughly translates to “terrible radiation-surviving berry”.
This bacterium can endure radiation levels thousands of times higher than what would kill a human, shrugging off conditions that would sterilise most known life.
It has been found surviving not only in radioactive waste sites but also in the vacuum of space during experiments on the International Space Station.
"When the Earth formed 4.5 billion years ago, at first its surface was hellishly hot. Flowing oceans of magma covered the Earth's surface, creating an inhospitable environment for life. And yet, life arose as soon as the terrain cooled," says Douglas Vakoch, author and founder of METI (Messaging to Extra-Terrestrial Intelligence), an organisation researching communication with alien life.
"Once life arose on Earth, it spread everywhere. The same evolutionary principles that allow life to adapt on Earth should operate throughout the Universe, creating exotic alien lifeforms unlike anything that we can imagine."
Even if alien life is out there, part of the challenge is finding it. For example, if you dedicated each member of the human race to search for aliens, every person would have to explore 13,000 planets.
While that has remained challenging for years, scientists believe that it is becoming easier to find the signs that would show an alien world is inhabited.
“We’re in a much better position to discover it now than we were five years ago. Everything is going in the right direction for detection. Especially for the detection of techno-signatures (a measurable signature that shows evidence of past or present technology on a planet),” says Garrett.
“Advancements in computing, digitisation, and artificial intelligence mean that we are moving very rapidly towards alien detection because the technology improves so fast now.”
Vakoch agrees. "In the next 20 years, we'll have telescopes that can scan the atmospheres of planets that orbit stars as we search for signs of life," he says.
"When the European Space Agency's ARIEL mission (a plan to observe and study 1000 exoplanets) launches in 2029, we'll have much greater capabilities to discover life through the chemical byproducts that life inevitably gives off."
Why we might actually be alone
So if aliens likely exist out there, why haven’t we discovered them yet? And why have we never received any extraterrestrial visitors? Welcome to the Fermi Paradox.
Over lunch way back in 1950, physicist Enrico Fermi threw out an intriguing question to his colleagues about extraterrestrials: “Where are they?” If intelligent life is common in the galaxy, Fermi argued, then Earth should already have been visited many times over.
It was a question that cut straight to the heart of the matter – and one scientists are still wrestling with today. Entire libraries of theories have been written to explain it, but some think the real answer could be surprisingly simple: a Universe big enough to host life is also big enough to hide it.
"Life on Earth – especially the rise of macroscopic, complex life – required a series of extremely unlikely coincidences regarding the composition and nature of Earth and its surface," says Jason Wright, a professor of astronomy and astrophysics at Pennsylvania State University.
“After all of that, Earth then required a series of extremely unlikely evolutionary innovations to occur. Those odds are already pretty small, but then you throw in the fact that the nearest aliens could be very far away.”
In other words, if life did exist elsewhere, we might just never be able to find it. Humans have inhabited Earth for a couple of hundred thousand years, which, in the grand scheme of the Universe, is short. We’ve only been around for 0.000002 per cent of its entire history.
Other intelligent civilisations could have come and gone, or are in the early stages of development. For us to discover life outside of Earth, many events need to occur at the same time.
“Science is inherently sceptical. We want proof – and so far, there’s no direct proof of life beyond Earth,” says Vakoch. “But a few decades ago, the same was true for planets orbiting other stars. We thought they might exist, but we couldn’t see them.”
That changed in the 1990s, when the first exoplanets were confirmed. Today, astronomers have discovered thousands, proving that other worlds are not only possible but abundant. “To find them, astronomers first had to believe they were out there,” Vakoch adds. “Then they did the hard work of uncovering them.”
For many researchers, alien life feels like it’s at the same stage exoplanets once were: strongly suspected, but still unseen. The tools to detect it are improving, but the breakthrough could take decades – or longer.
But who knows, maybe somewhere billions of light-years away, on an incredibly distant planet, an intelligent lifeform is sitting contemplating the thought: "Are we alone in the Universe?"
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