Nasa?s deployment of a space telescope to gaze beyond our solar system provides the best chance so far of detecting alien organisms ? intelligent or otherwise, writes Clive Cookson.

A spacecraft launched from Florida offers the best hope yet of answering a question people have been asking for aeons: Are there other worlds like ours or are we alone? Nasa?s $600m (?425m, ?478m) Kepler space telescope was dispatched from Cape Canaveral to search the galaxy for planets that could harbour life. The results, says Debra Fisher, astronomy professor at San Francisco State University, ?will help us chart a course toward one day imaging a pale blue dot like our planet, orbiting another star?.

The search for extra-solar planets, or exoplanets, has become astronomy?s hottest activity. Since 1995, telescopes on Earth and in space have detected 340 planets elsewhere in the galaxy. ?We know already about an incredibly wild and chaotic range of planets,? says Prof Fisher. The main point of the planet search, however, is not to find weird worlds in strange orbits with bizarre geology. What everyone wants to know is whether there is life beyond the solar system. If so, its discovery will be among the most important ever made. It would be the last step in humanity?s scientific demotion over the past few centuries, by Copernicus, Darwin and others, from specially created beings at the centre of the cosmos to just one of myriad life forms in the universe.

Even if the search for extraterrestrial life is unsuccessful, it is likely to bring benefits closer to home, in the form of new ways to discover and study micro-organisms on Earth. All exoplanets discovered so far are too extreme to support life as we know it. Kepler ? named after Johannes Kepler (1571-1630), who discovered that planets travel around the sun in elliptical orbits ? is the first observatory with technology capable of finding Earth-sized planets that are neither fireballs nor snowballs: worlds with liquid water and rocky continents.

William Borucki, the mission?s chief scientist, says it will survey 100,000 sunlike stars over the next four years, looking for orbiting planets in the ?habitable zone? ? sometimes known as the ?Goldilocks zone? ? where conditions ?are not too hot and not too cold but just right? for life. If Earth-sized planets are common, Kepler will detect hundreds of them, including dozens in the habitable zone. ?If we find that many, it will mean that life may well be common throughout our galaxy,? he adds. ?If we do not find any, that would be another profound discovery?, implying that Earth is a lonely outpost of life.

Most astronomers are optimistic, given the latest theories of planetary formation and the number and variety of giant planets already discovered. ?We already know enough to say that the universe is probably loaded with terrestrial planets similar to Earth,? says Alan Boss of the Carnegie Institution for Science in Washington. Boss believes that most stars similar in size and temperature to our sun are accompanied by one or more habitable planets. Given that there are an estimated 100bn sunlike stars in our Milky Way galaxy and 100bn galaxies in the known universe, the potential number of Earth-like planets is a humbling 10 to the power of 22 (one followed by 22 zeros).

Paul Davies, cosmology professor at Arizona State University, says there has been a huge change in attitude during his career. ?When I was a student in the 1960s, no one believed that there was life beyond Earth,? he says. ?Recently the pendulum has swung in the opposite direction, towards a universe teeming with life.?

Enthusiasts for exobiology ? life beyond Earth ? are encouraged not only by the plethora of planets around distant stars but also by the recent discovery that interstellar space is rich in organic chemicals. An intensive search with radiotelescopes has identified 150 molecules by their spectroscopic ?signatures?; these include complex chemicals such as alcohols, sugars and amino acids, which are precursors of life.

The origin of these ?prebiotic? molecules is mysterious. Anthony Remijan of the US National Radio Astronomy Observatory says they may form on the surface of interstellar dust grains, heated just enough by nearby stars to catalyse chemical reactions. Once formed, prebiotic chemicals could reach the surface of planets, where a process of proto-evolution would lead the molecules to organise themselves into increasingly complex self-replicating forms.

Yet the fact remains that we have evidence for life having started only once in the universe: On Earth 3.5bn-4bn years ago. Within our solar system, hopes remain high that, within the next decade, robot missions to Mars will prove the existence of past ? and perhaps of present ? microbial life. Later missions might possibly find that some moons of Jupiter and Saturn harbour life too.

Prof Davies also raises the possibility that life started independently on Earth on two or more occasions. ?Has there been a blind spot to the possibility of ?alien? life on Earth?? he asks. ?Might it exist on Earth today in extreme environments and remain undetected because our techniques are customised to the biochemistry of known life?? He advocates a systematic search on Earth for micro-organisms with an alternative biochemistry to the DNA, RNA and proteins that control all known forms of life from viruses and bacteria to humans. ?If we found life happened more than once here, then it means that the universe really is teeming with life,? says Prof Davies.

Beyond Earth and the solar system, there is no realistic possibility of making direct physical contact with alien life. So astronomers will use an increasingly sophisticated series of indirect means over the next two or three decades to look for it. The Kepler space telescope represents the first step. It will find Earth-like worlds by detecting tiny differences in the light coming from distant stars as orbiting planets pass in front of them. The light variations during these ?transits? are equivalent to an insect flying across a powerful searchlight beam ? previous observatories have not been powerful enough to record them.

Astronomers can deduce a surprising amount of information about a planet by observing a series of transits: its size, from the way its parent star changes brightness; its orbital period, from the time between transits; its orbital dimensions, from the star?s mass and size; and its surface temperature, from its orbit and the star?s temperature. This will tell whether the planet is potentially habitable but not, of course, whether it is inhabited.

After three years of Kepler observations, Nasa expects to have a reliable estimate for the number of Earth-like planets in our galaxy. Assuming that those are reasonably plentiful, space agencies can then plan the next stage: a mission to image planets directly ? possibly showing clouds, continents and oceans ? and analyse their atmospheres for chemical signatures of life, such as oxygen, methane and water. Although the European Space Agency has such a mission on the drawing board, called Darwin, and Nasa is thinking of something similar called Terrestrial Planet Finder, these are unlikely to be funded, developed and launched in less than 15 years.

The whole process could be short-circuited, however, by a dramatic coup de th颴re: The detection of signals transmitted by intelligent beings elsewhere. The search for extraterrestrial intelligence, known as Seti, started in 1960 at the Green Bank radiotelescope in West Virginia, under the direction of the American astronomer Frank Drake. The failure to hear a peep from ET after almost 50 years has not discouraged advocates of Seti, who say we have not listened long enough or hard enough ? or in the right way.

Nasa had trouble funding Seti in the face of ridicule from politicians such as the late Senator William Proxmire, a scourge of what he saw as frivolous public spending. But private donors stepped in to fill the gap, notably Paul Allen, co-founder of Microsoft. He has contributed $26m to the construction in California of an array of 350 radio dishes, each 6 metres in diameter, dedicated to listening for extraterrestrial signals.

Until now, Seti has had to beg for time on radiotelescopes designed for other purposes. Estimating how many intelligent civilisations exist in our galaxy is not much easier today than it was when Drake published an equation for the purpose in 1961. ?The numbers that came out of the Drake equation then were between one and 50m,? says Duncan Forgan, an Edinburgh university astronomer who has just carried out a new calculation, based on the latest knowledge of planetary formation and biological evolution. Prof Forgan estimates that between 360 and 38,000 intelligent life forms capable of interstellar communications have evolved over the lifetime of our galaxy so far ? but he says no one can put a sensible figure on the number of civilisations in existence now, let alone the chance of picking up a message from one of them.

Even if the Kepler mission gives a good estimate of the number of habitable planets, we will still have little idea of the chance that life will actually start, let alone evolve into a civilisation transmitting radio waves or laser pulses. Perhaps the biggest uncertainty is how long an average civilisation exists before it is destroyed. Or, more optimistically, perhaps some civilisations have become too technologically advanced for primitive Earthlings of the 21st century to detect signs of their existence. Astronomers still have little idea of the identity of the mysterious ?dark matter? and ?dark energy? that make up 95 per cent of the universe. Could aliens use them to communicate? Could they even exist in dark matter?

If we do hear from ET, elaborate diplomatic protocols will clank into action on Earth, to control how the momentous discovery is disseminated. Any reply to the aliens would have to wait for an international agreement. Although some astronomers are surprised that Seti has detected no signs of intelligent life elsewhere, they do not expect an alien visit to Earth. Short of a ?wormhole? or some other propulsion system beyond the ken of contemporary physics, the timescales required for interstellar travel are too vast; it would take several millennia to travel a few light-years to our nearest planetary neighbours. The energy required would be prohibitive, too. ?Actual visits would be ruled out by the laws of physics,? says Boss. Realistically, we can expect to know within 20-30 years whether there is life of some sort beyond Earth. As for intelligent life, we may have to wait an eternity.

The meaning of life is of increasing interest to scientists as well as philosophers. As the search for extraterrestrial life gathers pace, the exobiologists who specialise in this are studying the potential range of chemical and physical characteristics in living systems. Silicon-based life has featured in science fiction but most chemists say only carbon-based systems such as on Earth would be versatile and robust enough to evolve elsewhere.

These would have to follow the rules of organic chemistry - and of Darwinian evolution. ?Whatever its shape or chemistry, there is one important expectation that many if not all biologists share about life in the universe,? says Sean Carroll, professor of genetics at the University of Chicago. ?Wherever life has arisen, it has evolved by the two principles Darwin formulated: By descent with modification and natural selection.?

A universal requirement for life, then, is a biochemical store of genetic information passed down through the generations, with small changes to make evolution possible. On Earth this role is carried out by the DNA and RNA molecules, which carry data on chemical letters called nucleotides. Although scientists have not thought up a completely different genetic system, they have developed alternative forms of DNA incorporating synthetic nucleotides.

Last month at the American Association for the Advancement of Science meeting in Chicago, Steven Benner announced the first laboratory experiment in which artificial DNA underwent Darwinian evolution. ?If you are looking for alien life, it is helpful to have an alternative system to study,? says Dr Benner of the Foundation for Applied Molecular Evolution in Florida.

Instead of carrying out ?wet? experiments in a biochemistry lab, a different approach uses computers and software to simulate the creation of life under various conditions elsewhere in the universe. On an Earth-like planet, with oceans, continents and climate broadly similar to ours, any advanced life forms would probably not look wildly different to their terrestrial counterparts. For example, the history of life on Earth shows that features such as eyes and ears, which give organisms a huge competitive advantage, evolved independently on several occasions. It is conceivable, however, that any super-advanced civilisation could be unrecognisable to ours, if biological organisms have melded with machine intelligence to form some sort of hybrid biocomputer life form. The concept is controversial, but some futurists such as Ray Kurzweil predict that something like it will happen on Earth later this century.