There’s no easy answer to the question of alien life.

Aliens have captured our imagination for decades. We’ve seen them in movies, read about them in science fiction novels and killed them in computer games. Some people even claim to have met them. But what are the chances intelligent life actually exists beyond Earth?

Well, if we take the only example we have – Earth – it seems probable that extraterrestrial lifeforms have developed.

That’s because we have fossil evidence of basic life on our planet from 3.5 billion years ago – just 500 million years after the earth became habitable. And by the time Earth had formed, the universe was celebrating its seven billionth birthday. Many alien civilizations could have risen, mastered space travel and colonized their galaxy before we discovered fire!

So, the time frame seems to check out – but what about habitable planets?

So, the time frame seems to check out – but what about habitable planets?

On the face of it, this doesn’t seem problematic either. It’s estimated that 20 percent of all stars have Earth-like planets orbiting them in the Goldilocks Zone – a region capable of sustaining life because it’s not too distant from its star to be an icy wasteland, but not close enough to fry its inhabitants. 

Let’s put that into perspective. There are roughly 200 billion stars in our galaxy, the Milky Way. This potentially gives us forty billion Earth-like planets just in our cosmic neighborhood. 

But if alien life seems so plausible, why haven’t we been visited?

One theory argues that alien life might be common, but intelligent life is exceptionally rare. 

Let’s look at Earth again. Life took 2.5 billion years to go from single- to multi-celled organisms, which are needed for intelligent life. This is a significant chunk of the time we have available before our sun explodes. So there could have been many other life-sustaining worlds developing, only to be blown up by a grumpy old star. 

Let’s look at Earth again. Life took 2.5 billion years to go from single- to multi-celled organisms, which are needed for intelligent life. This is a significant chunk of the time we have available before our sun explodes. So there could have been many other life-sustaining worlds developing, only to be blown up by a grumpy old star. 

This isn’t alien life’s only existential threat: 66 million years ago, a small asteroid or comet slammed into Earth. It wiped out all of the dinosaurs – our planet’s previous dominant species. 

This was Earth’s last major impact, and we’ve been pushing our luck for a while now. A reasonable estimate for these collisions is around once every 20 million years. If this is true, it might be just down to good fortune that human life has developed on Earth, as our planet is long overdue for an interstellar collision. Other lifeforms might not have been so lucky.