"I think we're alone now"

Nick Morrison speaks to one man who says he knows the answer.

"INNUMERABLE suns exist; innumerable earths revolve around these suns in a manner similar to the way the planets revolve around our sun. Living beings inhabit these worlds." This may be a commonly enough held view today, but four hundred years ago it was enough to condemn its author to death.

Sixteenth century Italian monk Giordano Bruno was burned at the stake for his heresy. At a time when the Church taught that the Earth and its inhabitants were unique, to suggest that we might not be alone was very much beyond the pale.

Today, while the stakes may not be quite so high, the question of life on other planets has not lost its ability to fascinate and beguile. And the dominant feeling seems no longer to be that we are unique, but rather that we must be quite commonplace.

"I always start by asking people whether they think we're alone, or is there intelligent life out there which is able to communicate with us. About 80 per cent normally say we're not alone."

Professor Sir Arnold Wolfendale is one of the country's most eminent astronomers. The 14th Astronomer Royal, a post he held from 1991-5, he is Emeritus Professor of Physics at Durham University, and an expert on cosmic radiation. He also lectures across the country on the search for intelligent life - including a talk at Teesside University tonight - and it is at the start of these lectures that he poses his question to the audience.

"The usual argument is that there are so many millions of stars, almost certainly most of them have planets, so you think that as we're on a planet going around an ordinary star, the sun, we must be replicated all over the show," he says.

But while this may seem like a fairly common-sense approach - that the chance of us being alone must be pretty small - it is not quite as simple as that, says Sir Arnold.

"The easiest way of detecting intelligent life would be to pick up radio signals," he says. "Just as the Earth has been transmitting television signals for 60-odd years, one presumes that other civilisations will also be transmitting."

Despite the best efforts of Jodrell Bank and numerous other radio telescopes, the only communications snaking their way through space that we know of are the likes of Popstars: The Rivals and Celebrity Big Brother, unaccompanied by their alien equivalents.

But just because we haven't picked up anything else, it doesn't mean there isn't intelligent life elsewhere, surely? Particularly when it seems almost certain that life, in some form, does exist on other planets. Tests on meteorites thought to come from Mars show signs of fossils, meaning there was once life on the Red Planet. And not just the Red Planet, says Sir Arnold.

'In fact there is probably elementary life everywhere. If you can get it on Mars, which is not a very hospitable place, then you could get it everywhere. It is probably all over, it is just a question of the appropriate chemical mix, together with the appropriate conditions of light and oxygen."

But first, there is the question of how widespread it might be. It might be fairly common for planets to have some form of life at some time in their history, but if there are not many planets in the first place, then life is not exactly everywhere.

"Probably most stars have planets going around them. What we don't yet know is how many of these planets are hospitable to life. About 100 planets have been detected already, and every week a few more are found, so it seems a very common occurrence," says Sir Arnold.

It seems that gravity condenses the balls of dust and gas swirling around in space into discs - the stars - but these discs are unstable, and bits break off and become planets. Outside of our solar system, astronomers don't actually see the planets themselves, but infer their presence from the way some stars wobble, the effect of a planet's gravitational pull as it orbits its own sun.

Sir Arnold also suggests that the window for contacting - or being contacted by - life on other planets is quite sizeable. Although the Earth may be struck by comets, he reckons we're probably safe for around a billion years before something cataclysmic happens.

"If elementary life is probably common and we're probably safe for a billion years, you would think that intelligent life is probably common, because it has taken us only a few million years to get where we are," he says.

Eventually, of course, every star will die as it runs out of fuel. Our own sun is estimated to be around four-and-a-half billion years old, with another four-and-a-half billion years to go, but it will start to heat up in around a billion years, forcing us to move or die. By then, technology may mean this is no big deal, but this throws up its own challenge to those who think we're not alone.

"If life was common, then those beings would have had to go off in their spaceships and fairly soon would have come here - they would have colonised the galaxy. This is the crunch - because there is no evidence whatsoever of any alien life having visited us, you can argue that is because they are not there and we are unique," he says.

But there is an alternative answer, although it may not be a palatable one. That is that intelligent life may indeed be common, but it does not go on developing indefinitely. On the contrary, it may not go on developing much further that the stage we're at now, as Sir Arnold explains.

"When intelligent life gets to our level, it may have only another few hundred years to go. If that happens, and their suns are snuffed out, they won't be able to colonise anywhere else. There may be intelligent life all over the show, but if it is at our stage, it has not got long to go."

Sir Arnold is no eco-warrior, but it is not hard to see the ways in which we might foul up the planet, with global warming or polluting our water supply beyond repair.

But as his start-of-lecture poll shows, the instinct for believing that we are not alone is a strong one. With a billion stars in the galaxy, surely the odds are that more than one of them is someone else's sun.

But the odds against intelligent life may be higher than we think. If we are indeed alone, and Sir Arnold will reserve his answer to the question for his lecture, then one reason may be in our sun's position at the edge of the galaxy, keeping it out of the way of a mass of exploding stars which could have snuffed life out before it had really got going.

Our position in our own solar system may also have helped, with the giant planet Jupiter coming to our rescue in giving passing comets a gravitational kick, sending them spinning out of our solar system and away from the Earth. It may be that every planet capable of supporting life needs such a henchman before life can develop.

And without the misfortune that befell the dinosaurs, wiped out by comets 65 million years ago, we would never have evolved beyond little mammals whose only aim in life was to avoid being squashed by dinosaur feet.

"Whatever way you look at it, there are all sorts of funny coincidences that allow us to be here. There may be many, many factors that have led to our being here," Sir Arnold says. It may be a little disconcerting for many of us, and not just Giordano Bruno, to think that we are unique, but maybe that's just something the rest of us will have to live with.

* Sir Arnold will be giving a free talk on the search for intelligent life on other planets at room OL1, Europa Building, Woodlands Road, Middlesbrough, tonight, 6.30-7.30pm.