Are we unique, or is there life elsewhere in the universe? Geoff Watts surveys the scientific thinking

Definitive evidence of life on Mars. That was the confident claim made in early 2001 by Imre Friedmann of the US space agency Nasa's Ames Research Centre. It followed new studies of a Martian meteorite known as ALH84001. But not everyone shares Friedmann's confidence.

What he and his colleagues have shown is that ALH84001 contains chains of crystals of a material called magnetite. Some bacteria on Earth have similar crystalline chains that, because magnetite is weakly magnetic, they can use to orient themselves in the mud in which they live. The ALH84001 chains have characteristics that, say microbiologists, point to a biological origin.

On this the critics agree. The question, they say, is when and where the chains got into the meteorite. ALH84001 has spent the past 13,000 years lying where it landed in Antarctica. Perhaps it became contaminated with Earthly bacteria during that time. Friedmann refutes this. The chains, he says, were sealed in material that could only have come from Mars, and bacteria of the kind that use magnetite are not found in the Antarctic. The argument continues.

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For anyone who likes simple answers to straight questions, this dispute is depressing. If scientists cannot even decide whether life once existed so close to us in our own solar system, how will they agree if there is or was life elsewhere in the galaxy and beyond?

The astrobiological approach followed by Friedmann et al relies on searching for direct evidence of living systems. But it is not the only route. The best-known alternative is Seti, an acronym derived from the search for extra-terrestrial intelligence. The Seti Institute reasons that if life does exist elsewhere, it may have become at least as advanced as we are. If so, it will have developed radio-transmission technology. Just as Earthly broadcasters are inadvertently beaming The Archers into far infinity, so civilisations elsewhere must be disseminating their equivalent of our soap operas. Seti researchers therefore scan the electromagnetic spectrum looking for non-random signals arriving from space. Thus far, the search has gone unrewarded.

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With so little evidence, it is not surprising that the question of extra-solar life has been dominated by theory and speculation. The universe is made up of billions of galaxies. Our own, the one we affectionately call the Milky Way, itself comprises more than 100 billion stars. Putting to one side any religious claims that a deity has selected our particular planet as the only place fit for life, there is a case on grounds of sheer probability for suggesting that humans are not alone.

In his classic science-fiction novel The Black Cloud, Fred Hoyle envisaged intelligence forming within a highly organised cloud of gas. But organisms as tangible as we are are still the most likely possibility. Terrestrial life relies on the extraordinary capacity of carbon atoms to combine and recombine with themselves and with other elements in myriad different ways. Not many atoms share carbon's versatility, so it would not be surprising if extraterrestrial life relied on chemistry similar to our own.

Life elsewhere would also need an environment able to sustain it - not too hot, not too cold. It therefore seems likely that the bodies most likely to support life would be objects similar to Earth: solid, "temperate" planets in orbit around a star. We know that planets exist because we see them in our own solar system. And in the past decade or so we have found direct evidence of many more.

Astronomers now claim to have identified upwards of 50 planets orbiting stars other than our Sun. Most, like Jupiter, are large and gaseous and probably would not support life. But at the annual meeting of the American Association for the Advancement of Science in 2001, Norman Murray of the University of Toronto presented new evidence of the existence of large numbers of solid, more Earth-like planets in our galaxy.

In their sample of more than 400 stars, Murray and his colleagues reckon that more than half might have their equivalent of Earth. This is not to say that life would necessarily have appeared on them. But Murray is prepared to say that life could be common in the galaxy.

Some scientists go much further. They believe it is not just possible but highly likely that life has appeared elsewhere. Assuming that the behaviour of matter and the laws of physics are universal, they argue that the emergence of a system of molecular organisation of the kind we call life is inevitable. Others go further still: if such a process starts, the forces that resulted in evolution by natural selection on Earth would inevitably produce intelligent life. But how often?

In 1961, with a confidence verging on hubris, the US astronomer Frank Drake devised an equation for calculating the number of technological civilisations in our galaxy. The Drake equation can be expressed thus: N = R x ? p x n E x ? l x ? i x ? c x L .

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N is the figure he was trying to calculate - the number of civilisations in the galaxy that have developed to the point of being able to communicate. R is the rate at which suitable stars are formed - suitable, for these purposes, means likely to form planets. The next term, ? p , represents the proportion of stars with planets, while n E is the number of planets round any star with a habitable temperature range. The three ? factors represent the proportions of planets on which life evolves ( ? l ), reaches the stage of intelligence ( ? i ) and develops a communications technology ( ? c ). Finally there is L , the length of time for which an intelligent civilisation can hope to survive either accidental destruction by outside forces or self-destruction through misuse of its own technology.

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The Seti people's attempt to fit numbers into the equation puts the rate of star formation at about 20 a year. With a rising degree of arbitrariness, they suggest that half of all stars will form planetary systems, that the number of planets in a system that could support life is one, and that on one in five such planets life will appear and evolve. Mindful that whales and dolphins are intelligent but have never developed technology, they suggest that technology might be expected to appear in half the other worlds that support life.

Putting these numbers into the equation you get: N = 20 x 0.5 x 1 x 0.2 x 0.5 x L. That is, N = L . Or, to spell it out, the number of civilisations in the galaxy is equal to the number of years ( L ) that an advanced technological civilisation can hope to endure. The only such civilisation we have to go on is, of course, our own, which has been seriously technologically advanced for only some 50 years. So, the number of advanced life forms in our galaxy is 50 - at least.

This, of course, is just the Seti Institute's calculation. With assumption piled upon assumption, the Drake equation can be used to generate almost any result you like. However, Monica Grady of the British Natural History Museum, an authority on meteorites and the evidence of primitive life within them, says that scientists do take the equation seriously. "I do think it still has a legitimacy," she says. "It is setting the framework, the likelihood of an extraterrestrial civilisation. It gives us at least a back-of-the-envelope context."

She reckons the chances of life elsewhere are about 50/50 and, like others, she draws a sharp distinction between any life and intelligent life; the former could have appeared time and again without progressing to the latter. Of the search for intelligent life in particular, many scientists are sceptical. Astrophysicist Michael Rowan-Robinson of Imperial College, London, points out that all planets have a finite life. Sooner or later, the stars on which they rely for their energy supply will grow dim (still several billion years to go in our case). A very long-lived civilisation would have developed the technology to do more or less anything it pleased, so responding to the imminent death of its star would become the number-one global project. Such a civilisation would be keen to communicate with, if not to colonise, a new world. In other words, we should have had some inkling of its presence.

"People who are firm believers in the existence of intelligent life elsewhere tend to go all mystical at this point," Rowan-Robinson says. "They say things like, 'Ah, but they hide from us. They're able to communicate without our knowing.' I just don't find this convincing."

He accepts the argument that many civilisations reaching a certain point of development might tend to destroy themselves. "But you'd think that any civilisation that had been around a really long time would want to leave behind a monument," he says. "A beacon sending signals targeted at people like us. Of course it may be that when you're about to blow yourself up you don't have time to think of erecting your equivalent of the pyramids! All the same, I'm surprised that nothing's been found."

Should scientists ever discover firm evidence that intelligent life elsewhere does exist, it would not put an end to their search, of course. It would be just the beginning.

The Big Questions in Science is published on October 3 by Jonathan Cape, ?15.99.  

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It's worth another visit to our next-door neighbour to look for signs of life

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