Atomic, Molecular, Optical and Positron Physics


Is anybody out there?

Is there anybody out there? Astronomers think the answer may be blowing in the wind, writes Jonathan Tennyson

Guardian, Thursday February 8, 2001

Seen any UFOs recently? No? Well I haven't, but then I am a scientist - a breed notoriously impervious to this brand of visitation from "little green men" or perhaps I should say "LGM", as they are known in academic circles, maybe to hide the implicit sexism. But in academe, things are stirring on the LGM front. While reported visitations are still received with scepticism bordering on hostility, the feeling that we are not alone in the universe is widely accepted. More intriguing is that the possibility of spying on LGM in their own homes is becoming a real possibility.

Before anyone rushes off to their travel agents to book a trip to a truly unspoilt holiday destination, I should stress that it is voyeurism, not visits, that I am talking about. Up until fairly recently it was a matter of fierce debate among astronomers as to how many billions of other stars we could see that also had planets orbiting them. Or indeed whether any of them did.

Then a few years ago people started observing such planets mainly by the slight wobble they caused to the motion of their own sun as they orbited it.

At first, such sightings were surrounded by controversy. Some of the original claims had to be withdrawn because of difficulties in interpreting the observation. But now there is an increasing body of confirmed sightings, even round stars which seemed highly unlikely candidates to support planetary systems. Planets seem to be ubiquitous.

But how does that help with finding LGM? The late astronomer Carl Sagan was a great man, not only because of his ability to communicate the excitement of science to many, but also because of his occasionally off-beat scientific ideas. It was Carl Sagan who persuaded Nasa to do a truly stupid scientific experiment: could they detect signs of life on Earth?

As the Galileo space probe was on its protracted voyage to Jupiter, Sagan got Nasa to turn its telescopes towards our planet and tell us what they saw. Among all the clutter of "civilised" 20th century life: radio and TV signals and so forth, Galileo detected one feature loud and clear. This feature was methane, sometimes known as marsh gas.

Why is a signal of methane so important? In an atmosphere full of oxygen, like the one on earth, methane should rapidly be destroyed to form carbon dioxide and water. It is present in our atmosphere in significant quantities because it is constantly being produced not only by decaying vegetation but also by "flatulent rumenoids", as the journal Nature so tactfully described it.

Of course, there is no great technology in cows farting. Perhaps even more importantly a brontosaurus could undoubtedly let rip to good effect. Life on Earth has been visible in this fashion for many millions of years.

This distinction in timescales is important and one which makes the search for extra-terrestrial intelligence (Seti) so difficult. Earth has been emitting radio signals for less than a century, not even a blip in the lifetime of the universe. Against 10 billion years, a hundred million years or so of methane is still short but not impossibly short. One in a 100 are long odds, but not impossibly long, particularly if there are lots of planets to look at.

Does this help us find LGMs? Well not immediately as, at the moment, the newly discovered planets cannot be seen against the bright, bright stars they orbit. But telescope technology is evolving fast: already astronomers are laying plans for detecting these tiny specks of planetary light by cancelling out the huge flaming background.

Once this can be done, in maybe a decade or two, it will only be a short step to look for the tell-tale signs of oxygen and methane. We may then not actually be able to see inside the LGM's house, but should know whether he or she had beans for dinner!

• Jonathan Tennyson is professor of physics at University College London.