Five reasons why we haven't found aliens yet

As Prof Stephen Hawking and colleagues set out on the biggest alien hunt ever, we take a speculative look at why we haven't already found any little green men (or extraterrestrial microbes, for that matter).

We are unique. Hurray.

Arecent studyclaims that we haven't heard from aliens because they're already dead. "The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," says lead author Dr Aditya Chopra. "[But] early life is fragile, so we believe it rarely evolves quickly enough to survive." The scientists argue that life on other planets would become extinct before it had time to create a balanced atmosphere where greenhouse gases are kept in check. "Most early planetary environments are unstable,” says Chopra.

In the world ofStar Trek, the extraterrestrial Vulcans don't bother getting in contact with the humans, because they find them irrelevant and dumb. It's not until Zefram Cochrane invents a warp drive (a device that lets you go faster than light) that they change their minds. So maybe aliens simply don't care about our existence because we're stupid and they can easily hide from us. Again, because we are stupid.

Observations from NASA’s Hubble Space Telescope and the planet-huntingKeplerspace observatory suggest that our Earth was one of the first ever habitable planets. “Compared to all the planets that will ever form in the Universe, the Earth is actually quite early," said Dr Peter Behroozi back in the December 2015 issue ofFocus. So maybe life just hasn't had time to get started on other planets.

Some astronomers think that we've got the wrong idea when it comes to looking for extraterrestrials. Astronomer Royal Lord Martin Rees is known for having said that aliens are more likely to be machines - not organic creatures. In this case we should be looking for pollution and solar-harvesting structures instead of listening for radio signals.

So there are reasons why E.T. has been thus-far absent. But for many, it's just a matter of time. Maybe theBreakthrough Listenproject - the largest ever hunt for alien life - will be the first to strike gold.