Hello, ET? Um, I’m at a loss for words

We are the first generation of serious alien hunters, according to planetary scientist Sara Seager of the Massachusetts Institute of Technology.

“We are finally on the verge of being able to search for signs of life beyond our solar system around the nearest hundreds of stars,” Seager said this week – and she thinks we might find it in the next two decades.

But are we ready for that? Astronomer Jocelyn Bell Burnell doesn’t think so. Like Seager, she anticipates “first contact” in the next 20 to 100 years, but she recently declared us culturally unprepared. What, for one thing, would we say to them?

Given the separation of space – it takes more than four years for a radio signal to reach even the nearest star – we needn’t rush into a decision.

But the discovery of alien life is unlikely to lead to a conversation, not even one stilted by a decades-long time delay. The search for extraterrestrial intelligence (Seti), the collective name for a number of activities that scan the skies for messages encrypted in radio broadcasts and the like, has so far found not a whisper.

Indeed, almost the only flurry of excitement came when Bell Burnell discovered pulsars in 1967: the regular lighthouse radio beam of collapsed, rotating stars was initially suspected of being a purposeful signal. Seti has been rightly compared with the Victorian enthusiasm for spiritualism: an attempt to communicate with invisible simulacra of ourselves, in a quest motivated by cosmic loneliness.

No, if we find aliens in our lifetime it most probably won’t be from their intentional messages but, as Seager explained, from inadvertent traces they leave in their planetary atmospheres. About 1 800 “exoplanets” (those that orbit other stars) have already been discovered. These numbers received a huge boost from the launch in 2009 of Nasa’s Kepler space observatory, but the James Webb space telescope, the successor of the Hubble, will take the search to a new level after its scheduled launch in 2018.

Peculiar mixtures
The hope is that life will reveal itself from peculiar mixtures of these planets’ atmospheric components: chemical blends that can’t conceivably be produced or sustained by purely geological processes. Any intelligent alien gazing on our own atmosphere would surely conclude that the planet is inhabited: the large amount of oxygen, a highly reactive element, makes no chemical sense unless it is constantly being produced, and photosynthesis by plants and bacteria is pretty much the only way we know that does that constantly on a planetary scale. Ruling out geological sources is not always easy, but the central idea is that posited by James Lovelock for the Mars Viking lander missions in the 1970s: life creates chemical disequilibrium in the atmosphere.

This is exciting stuff, but ever since the first moon voyage fantasies of the 17th century, we have imagined alien life in our own image – if not necessarily human, then at least terrestrial. The importance placed on finding water on other worlds, for example, has no other rationale than that life on Earth can’t exist without it. The occasional excitement at the discovery of seemingly “Earth-like” exoplanets is understandable, but the fact is we have no idea what we’re looking for, unless it is for ourselves.

Not only is there no rigorous, scientifically agreed definition of “life”, there probably never can be. Like “music” or “culture”, the meaning depends on its fuzzy boundaries – try to pin it down and it evaporates.

But this very lack of definition is ultimately one of the best reasons to be excited about the extraordinary diversity of worlds that our

telescopes are revealing. Above all else, they will force us to broaden our imagination about what life can be, and how we might recognise it. – © Guardian News & Media 2014

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