Does a wobbly star warm ET?

Most people believe that there must be life beyond the earth. Colin Pillinger surveys what scientists are doing to discover if we are not alone During the media excitement following Nasa's mid-1996 announcement that some of its scientists had recognised what they believed were tiny fossils in a meteorite from Mars, I was asked how fundamental a discovery it was. Searching for an astronomical comparison, I responded that to discover we are not the only planet having a biology would be as radical as Copernicus's discovery that the earth goes round the sun rather than vice versa.

I am always saying that nothing is new in science (you only have to be the first to rediscover it), so it came as no surprise to find much the same analogy quoted at the beginning of Life on Other Worlds by Steven Dick - attributed to Otto Struve in 1962. Broadening his horizons just a little, Struve added that learning our solar system was not at the centre of the galaxy was just as significant. It is, of course, a matter of opinion which you would consider the greatest achievement: recognising where we fit into the scheme of things or how we came to be here to wonder about such questions in the first place. As Dick describes in his book, an awful lot of people have wondered from an astronomical, biological, chemical, physical, philosophical and even literary standpoint. Nasa's announcement simply drew attention to the perennial question yet again.

It would come as an enormous shock to me, and to more than 80 per cent of the population according to a survey, had life developed only once in the vastness of the universe, with all its galaxies and stars. Why should luck have it that such a unique event occurred in an insignificant backwater about 30,000 light years down some dim spiral arm of a galaxy called the Milky Way? As I see it, the elements of life - carbon, hydrogen, oxygen and nitrogen - are the most abundant in existence. To involve them in the necessary chemical reactions requires providing the right catalyst and initiation. But according to estimates of the probability of success in producing life given in this book, if you rely on chance alone then, even with all the suns and planets there might be and the great age of the universe, you could still be waiting a very long time. Bearing this in mind, there have been numerous appeals to panspermia both biological and chemical. One of the laboratory successes of our generation has been the progress made in producing more complicated molecules relevant to life from simple precursors, thus shortening the odds of success considerably.

For the average person standing outside at night looking up at the stars, the question is not about probability but about emotion. The figure of 80 per cent mentioned above refers to intelligent life; if we refer to any life form, the percentage of people believing that something living exists somewhere "out there" would probably rise much higher. I should say where the figure of 80 per cent comes from: it derives from a head count held during a discussion after a lecture I gave when Sir Arnold Wolfendale was in the audience. He is one of the few people I know who is entirely in the opposite camp regarding intelligent life. He would not let me frame the question about any form of life and could not be drawn on the subject of non-primitive life forms, so I do not know just how much of a minority he represents. Neither could I find the answer to the question in the lecture he gave to the Royal Institution, "On the search for extraterrestrial life", which forms an excellent opening chapter to the book of the same name edited by Peter Day.

I should say, though, that this second book is guilty of exploiting the public's insatiable thirst for information on the subject of extraterrestrial life to sell the text of eight lectures in the Royal Institution's Friday evening discourse series. While the lectures make very good reading - even if in places one can detect that the essays derive from lecture notes - seven out of eight bear no relation to the title of the book. How do television beyond the millennium, pondering on Pisa and Raman spectroscopy as a means of identifying pigments used in ancient manuscripts get into a book ostensibly covering a subject so profound that it challenges Copernican dynamics for precedence?

The nearest one comes to the putative subject matter is in the lecture by Monica Grady, who mentions Martian meteorites as an aside in her comprehensive survey of all that can be learned from meteorites. Next closest is Dan McKenzie, who gives an informative personal account of unravelling the planetary geology of Venus, as gleaned from the Nasa Venus-orbiting radar experiment. But, he believes that Nasa's preoccupation with extraterrestrial life has been to the detriment of other planetary studies. I cannot agree. The search for the conditions and ingredients appropriate for life underpins space exploration by all the participating agencies (from the US, Europe and Japan). The advances in understanding comets and the moons of giant planets, and plans to look for other solar systems, can be identified as just a few of the spin-offs of the bigger goal.

If most of the subjects in The Search for Extraterrestrial Life would leave me feeling a trifle misled had I bought it, Life on Other Worlds has entirely the opposite effect. This subject has spawned any number of sensationalised books. Dick's is not one, but rather a thorough account of 20th-century thinking and quite a lot of earlier material. My main criticism is not of the author but of the publisher: surely nowadays we should have high-quality illustrations and paper to match the quality of the text? And here is slight niggle about the book's coverage of my own group's contribution to the Martian meteorite debate. We did report indigenous organic matter in one of these stones seven years before Nasa. We were more than pleased that our closing remark concerning the finding that "the implications were obvious" attracted 42 media inquiries to a press announcement released on a day when Sir Arnold Wolfendale was visiting my laboratory: our message was interpreted in the way we intended. When we held another meeting in October 1996, it was primarily to set the record straight about our 1989 contributions and to remind everybody that the Martian meteorite we studied was only 180 million years old and left Mars 600,000 years ago. Again, the implications are obvious.

On another point, work on Martian meteorites is continuing. For this reason, Dick may be unaware that the balance in the controversy over the temperature at which carbonates were deposited by water trickling through the rock that was to become meteorite ALH84001 has swung a long way in favour of those who advocate values appropriate to biology rather than volcanology.

But I am nit-picking. Life on Other Worlds is a book of real depth. It includes chapters on the search for extraterrestrial life, the UFO controversies that were all the rage in the 1950s and 1960s, and the role of extraterrestrials in literature and film. There are subjects here that everyone knows about, such as Orson Welles's exploitation of H. G. Wells's The War of the Worlds in his radio play of 1938. Likewise the contributions of Arthur C. Clarke, but less so the work of Olaf Stapledon, who inspired Clarke. Dick covers everything from super-aggressive to benevolent Martians. Wherever appropriate, he discusses the scientific rationale that might have motivated alien invaders, such as low abundance of water on Mars. Very early science-fiction writers seemed to have been remarkably well informed.

After the meteorite aspects of exobiology, one of the hottest topics around is finding Jupiter-sized objects orbiting stars other than our sun. This field has experienced a quantum leap since the observation of planetary disks and, in particular, after 1995 when a Doppler-shift method was used to identify the wobble induced in a parent sun because of satellite planets. Star wobbling is covered by Dick back to 1782 - the most convincing early case being that of Barnard's star, which inspired much work in the period after its discovery in the late 1930s. What we cannot yet find is terrestrial-like planets in the so-called habitable zone where water would be liquid most of the time over a majority of a planet's surface.

One subject Life on Other Worlds does not cover is the plethora of space missions being prepared or planned in the search for life. All those places where the existence of water has been indicated - comets, the south pole of the moon, Europa, Ganymede etc - are on the agenda, but these activities are for the 21st century and beyond the scope of this book. There promises to be a traffic jam around Mars in the next ten years with Nasa, Esa, Japan and even Britain's Beagle 2 arriving, if the consortium I lead is successful. Although 20 years ago, immediately after Viking , the possibilities of finding life on Mars were considered slim, there is more optimism now. Some are pessimistic about the prospect; a society was recently formed dedicated to ensuring that Nasa, in conjunction with its collaborators in the French Space Agency CNES, does not risk bringing Martian samples and any accompanying life forms directly to earth. This group advocates a period of quarantine in space, such is their conviction that some of the experiments of Viking proved positive. In respect of this great debate, we live in exciting times.

Colin Pillinger is professor of planetary science, Open University, and Gresham professor of astronomy.