Are we alone in the universe, or is earth a typical planet in a cosmos teeming with life? This age-old conundrum is back in the news, with the recent discovery of planets in orbit around the stars 51 Pegasi, 47 Ursa Major and 70 Virgo. Thrilling though these discoveries are, a planet is one thing, life quite another.

All speculation about alien life-forms is hampered by the absence of a credible theory of the origin of life. Discounting miracles, scientists still have no idea whether life on earth is a one-off freak occurrence or a more or less inevitable product of normal physical and chemical processes. Attempts to create life in the laboratory have served only to emphasise the staggering complexity of life on earth. If it arose in anything like its present form by accident, then the chances of such a convoluted quirk happening twice are infinitesimal.

On the other hand, nature exhibits remarkable examples of complex order in the inanimate realm. From snowflakes to hurricanes, from the rings of Saturn to the great red spot of Jupiter, organised complexity has emerged naturally and spontaneously. Perhaps there exists a universal principle of nature that drives matter and energy towards ever-greater complexity and organisation. If so, we might expect life to be as ubiquitous as snowflakes.

Similar sharp differences pertain to evolution. If earth were to be struck by a massive asteroid, destroying all but the microbes, what would we expect to find here in a billion years time? According to orthodox biology, Homo sapiens would not arise again next time around. Indeed, there is no reason to expect that intelligent life in any form would be rediscovered.

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Biologists insist that evolution is simply a genetic lottery - a purely random walk through the realm of all possibilities, devoid of any trend or directionality. If so, intelligence must be regarded as a sheer fluke, like eyebrows or coloured irises. In that case even if, against all the odds, life got started somewhere else, there would be no "progress" towards intelligence, no inevitability about the emergence of technology or communication.

Again, some heretics disagree. They suggest that nature, the watchmaker, may not be completely blind. Perhaps the same deep principle that has created the richness of galaxies, stars, clouds and crystals from the featureless subatomic soup of the big bang also drives evolution towards intelligence and technology?

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The fact is, we have no idea. Only by exploring the universe might we obtain the answers to these burning questions. Jean Heidmann is a distinguished French astronomer, long associated with the fringe subject of exobiology. In this broad survey, directed at the nonspecialist reader, he examines the physics, chemistry, biology and astronomy that will determine whether or not we are alone.

The search for extraterrestrial life proceeds at two levels. There is the study of the emergence of life - what conditions are necessary, what chemical steps might be involved, how long would those steps take, how plentiful might suitable stars/planets/chemicals be in space? Then there is the direct search for the products of intelligent life: alien artefacts or signals.

In the 1950s astronomers realised that their radio telescopes were capable of interstellar communication. Some astronomers, most notably Frank Drake in the US, began listening to likely nearby stars in case radio messages were being beamed at us. This needle-in-a-haystack quest was assisted by the expectation that any aliens signalling would presumably make it easy for us and pick a radio frequency that is in some way special. Drake and others thought the emission frequency of cold hydrogen gas, 1420 MHz to be a very suitable candidate.

Thus began Seti, the Search for Extraterrestrial Intelligence. Others joined in. Eventually the SETI Institute was founded in California, and the technology developed to scan millions of radio channels simultaneously. By the early 1990s a truly systematic search of the skies seemed at hand.

Unfortunately, at this point, Nasa pulled its funding plug, and the SETI Institute turned to private individuals for money. As a result of generous donations, Project Phoenix began in early 1995. It consisted of a five-month scrutiny of 200 nearby stars using the Parkes radio telescope in Australia. Nothing unusual was found, but the old adage "the absence of evidence is not the same as the evidence of absence" was never more true than for Seti. Even the most optimistic proponents of Seti concede that the chances of success with such a limited programme are very slim.

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Heidmann appraises Seti's bewildering array of options, undaunted by the cosmic odds. Which stars to target? Which frequencies to use, and in which reference frames to define them? Is it better to use narrow-band or broad-band radio? Should we expect laser signals rather than radio messages? Could we eavesdrop on alien domestic radio traffic?

Life on earth has taken four billion years to evolve as far as technology. The probability of another planet having life that just happened to reach technology at the same time as us is therefore infinitesimal. If intelligent aliens exist, they will almost certainly be far in advance of us, perhaps by millions or even billions of years. So what does a million-year-old civilisation have to say to us? Would they ever bother to get in touch?

There are no easy answers. We have no idea how intelligent aliens would think, or even whether they exist at all. What might motivate them deliberately to signal us? Curiosity or greed? Altruism or mischief? We have only Homo sapiens to go on, with scant idea as to how many of our prejudices are accidents of evolution and how many might be universal.

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Take language. How should we begin to communicate? Most experts think that maths is the key. After all, as Galileo remarked, the great book of nature is written in the language of mathematics. The universal mathematical laws that enable us to build and operate radio telescopes should be familiar to alien radio astronomers too.

Yet, is this merely being chauvinistic? Mathematics is the product of the higher human intellect. How much of our mathematisation of nature is due to our quirky evolutionary history, and how much of it reflects what is "really there" in nature? Might intelligent aliens see the world in terms of love, or poetry, or something outside our cultural experience altogether? Is it possible they are even now frantically trying to attract our attention in a way that fails to register?

Clearly Seti leads us into some very deep waters. We should not be put off by this, however. Whatever the obstacles presented, the search for extraterrestrial life should go on, if only because it forces us to confront the issues of who we are and what our place might be in the vastness of the cosmos.

Paul Davies is professor of natural philosophy, University of Adelaide, Australia

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