Extra-terrestrial beings. Big, black-eyed creatures from another world. Dark and foggy roads with no one around. Experiments and probes and… but wait, I don’t want to describe my plans, rather, I want to discuss aliens!

The idea that humanity is not alone is a very appealing one. We don’t want to think that in the entire universe, we are the only intelligent species here. So far, we have no evidence for the existence of intelligent alien life, and only flimsy evidence of any sort of alien life.

What evidence is  there for alien life?

Well, evidence is probably the wrong word. It was once theorised that life on earth began when rocks containing microbes was blasted of the surface of Mars in a collision and ended up crashing on earth. This theory is not currently seen as a major one, though it is still suggested that asteroids such as ALH84001 and Nakhla (known to come from Mars) contain fossils of microbes that originated on Mars.

It is thought that, under the icy surface of Jupiter’s moon Europa, a giant ocean of liquid water exists. Water is vital to life as we know it, so if there is indeed a huge underground ocean on Europa, it is a strong candidate to look for life.

Saturn’s moon Titan does not currently harbour life, but the conditions there are thought to resemble a primordial earth, and when our sun expands into a red giant, Titan may just receive enough heat to stir up these organic compounds into something more than they current are.

What about intelligent alien life?

At the start of this page, I played with the popular idea of big grey creatures with big black eyes. There is a popular narrative about beings like this abducting people and performing experiments on them, before returning them to earth with no memory of their experience, save for an unsettling feeling of something weird and scary happening. Whilst there are many documented cases of alien abduction, hard evidence to back them up is usually non-existent, and skeptics are swift to point out a number of flaws in the stories.

The same applies to alleged UFO (unidentified flying object) sightings. The vast majority of these have been shown to be planes, atmospheric disturbances or just plain hoaxes. Some do remain unexplained – but this does not automatically mean they are alien spacecraft!

It’s tempting – seriously tempting – to believe all the conspiracy theorists and the idea that governments are covering up alien encounters – but the truth is far more likely to be something mundane – like Area 51 is a research facility that doesn’t have alien bodies and wrecked UFOs.

So, the evidence for intelligent alien life visiting earth is either circumstantial, controversial, or just plain made-up.

But does intelligent alien life exist elsewhere?

We now know that rocky planets of a similar size to earth exist in orbit around distant stars – and as our detection methods are refined, we will find more and more extra-solar planets. It is inevitable that some of these will wind up in their star’s habitable zone – even one in fifty to do so will yield a tremendous number of planets that could support liquid water (a key ingredient of life). Sooner or later, we will be able to figure out if these rocky worlds have atmospheres, and what the composition of those atmospheres is. If there are alien civilisations out there, we should, eventually, pick up signs of this – but so far, despite listening for alien signals for several decades, we have not detected any. What does this mean for the probability of finding another civilisation like ours?

Well, projects to hunt for signals from ET are very limited in size and scope. The vast distance between stars also means that a signal could conceivably be on its way to us right now, but even if it came from our nearest neighbour, it would take four years to reach us.

So, we are not currently in a position to easily find or communicate with alien civilisations.

Will we ever find another civilisation out there, either space-faring or like ours now? I don’t know. The odds are good that such civilisations exist – the universe has billions of galaxies and each one has billions of stars, and even if ten percent of those has planets, and if only ten percent of those can support life, that’s still billions of potentially habitable worlds. I believe that there is intelligent life out there, and I hope that, one day, we will find it.

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