Our conceptions of alien life are based on a sample of one: Earth’s life. That means even our wildest imaginings are likely to be completely off beam

Bruce Murray/Alamy Stock Photo

LET’S get one thing out of the way: aliens are almost definitely out there. On average, every star in the Milky Way has a planet orbiting it. Fully one-fifth of those stars have a planet that could be temperate and conducive to life as we imagine it. That’s 50 billion potentially habitable planets just in our own galaxy – which is one of billions in the universe.

“If you’re going to say that there’s no chance we’re going to find any life elsewhere, you must think there’s something really miraculous about Earth,” says Seth Shostak at the SETI Institute in Mountain View, California. “And that’s a suspicious point of view, that we’re just miraculously better than all the other planets.”

That doesn’t mean intelligent life is close by. We have been exploring our solar system for a long time, so if it contained intelligent life forms we would probably know about it by now. With simple, microbial life, it is a different story. The best places to look are the icy outer solar system moons Europa, Enceladus and Titan because we know they have liquids that could support life, says Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University in New York.

“Our most basic assumptions about life are from a sample of one planet”

For anything bigger, we must peer further afield – and, as yet, our technology for spying life at a distance is rudimentary. Our best bet is to study the atmospheres of alien planets for signatures of gases like oxygen and methane that only coexist if some thermodynamically implausible process …