So where is everybody else living?

Either we on Earth are a freak accident, or the Universe is teeming with life. How close are we to finding out?

In the 19th century, Darwin and the geologists deduced that our Earth and its biosphere were the outcome of at least a billion years of evolution. We now view our solar system in a still grander perspective. Our Sun is just one of a hundred billion stars in our Milky Way, which is itself just one galaxy among billions of others. The whole cosmic tapestry can, we believe, be traced back to the initial instant of a Big Bang. But how special is our Earth in this vast cosmos?

In the 19th century, Darwin and the geologists deduced that our Earth and its biosphere were the outcome of at least a billion years of evolution. We now view our solar system in a still grander perspective. Our Sun is just one of a hundred billion stars in our Milky Way, which is itself just one galaxy among billions of others. The whole cosmic tapestry can, we believe, be traced back to the initial instant of a Big Bang. But how special is our Earth in this vast cosmos?

Astronomers have long suspected that other stars are orbited by planets, but firm evidence eluded us until a few years ago. Even now, the only planets that can be detected are very large ones - the size of Jupiter. But there's every reason to suspect that these are the largest members of other solar systems. Planetary systems are so common in our galaxy that Earth-like planets must be numbered in millions.

The US has a larger and more ambitious space programme than Europe's - a legacy of the Cold War, when superpower rivalry ramped up its aerospace industry to a larger scale than Europe's. The largest component of the National Aeronautics and Space Administration's programme is the hugely expensive manned Space Station - a project that is neither practical nor inspiring. But it's better news that Nasa's chief executive, Dan Goldin, has focused the less expensive unmanned effort on the so-called "Origins" programme - a series of projects aimed at probing the origin of stars and planets, of life, and of the entire Universe itself.

To the casual observer, stars have little individuality; they are remote points of light, distinguishable from one another only by their brightness and colours. But 20 or 30 years from now, the topography of the night sky will seem far richer and more interesting. We're all familiar with the layout of our own solar system - the sizes and orbits of the nine major planets. But astronomical surveys will reveal new planetary systems, orbiting hundreds of other stars. Nearby stars will no longer just be points of light. Many will be encircled by planets. We will know the orbits of each star's retinue of planets, and the sizes (and even some topographic details) of the bigger ones. Sky charts will be far more fascinating.

We'll be especially interested in possible "twins" of our Earth - planets the same size as ours, orbiting other Sun-like stars, and with temperatures that allow water neither to freeze nor to boil. Large telescope arrays in space will be able to reveal any Earth-like planets that may be orbiting our stellar neighbours.

Viewed from (say) 10 light years away - the distance of a nearby star - our Earth would be, in Carl Sagan's phrase, a "pale blue dot", seeming very close to a star (our Sun) that outshines it by many orders of magnitude. The shade of blue would be slightly different depending on whether the Pacific Ocean or the Eurasian land mass were facing us. By observing other planets, we can therefore infer whether they are spinning, and the length of their "day", and even infer something of their topography and climate.

Might these other worlds harbour life like our own? Life on Earth has occupied an immense variety of niches. The ecosystems near hot, sulphurous outwellings in the deep ocean bed tell us that not even sunlight is essential. We still don't know how or where life got started. Was it in Darwin's "warm little pond", or deep underground, or even in dusty molecular clouds in space?

The emergence of life may be a routine event in the cosmos; on the other hand, it may be hugely improbable. We just don't know. This is a question for biologists. But biology is a harder subject than astronomy; stars and galaxies may be vast and remote, but they are far less complicated than living organisms, with layer upon layer of intricate structure.

So long as we know only about our own biosphere, we can't dismiss the possibility that we are the outcome of a chain of events so immensely improbable that it happened only once in our galaxy. But if life emerged twice within our solar system, that couldn't be a fluke; we could then confidently infer that it must have commonly arisen on planets around other stars, and that the entire galaxy teems with life. That's why it would be so crucial to detect life, even in simple and vestigial forms, elsewhere in our solar system - on the arid surface of Mars, or in oceans under the ice of Jupiter's frozen moons, Callisto and Europa.

Only in the last four years of this millennium have we known for sure that "worlds" exist in orbit around other stars. But we are little closer to knowing whether any of them harbour anything alive. Still less do we know the likelihood that simple life would evolve into anything we could recognise as intelligent.

Systematic scans for signs of intelligence beyond our solar system are a worthwhile gamble, despite the heavy odds against success, because of the philosophical import of any detection. A manifestly artificial signal - even if it were as boring as a set of prime numbers or the digits of pi (the mathematical constant) - would convey the momentous message that "intelligence" (though not necessarily "consciousness") wasn't unique to the Earth and had evolved elsewhere, and that concepts of logic and physics weren't peculiar to the "hardware" in human skulls.

The odds may, of course, be stacked so heavily against intelligent life that there is none anywhere in our part of the universe. Some may find it depressing to feel alone in a vast, inanimate cosmos. But I would personally react in quite the opposite way. It would in some ways be disappointing if searches for extraterrestrial signals were doomed to fail, but if our Earth were the sole abode of life in our galaxy we could view it in a less humble cosmic perspective than it would merit if our universe already teemed with advanced life forms.

One thing we've learnt from astronomy is that the expanse of future time - even if not infinite - is far longer than even the 12 billion years that have elapsed since the genesis event that set our universe expanding. Our own Sun has burnt less than half its fuel, and will continue to shine for longer than it has taken for us to evolve from simple beginnings on the young Earth.

Even if life is now unique to the Earth, there is time for it to spread from here through the entire galaxy, and even beyond. However different they and their "world" were from ours, aliens would be made of the same kinds of atoms and governed by the same forces as us; they would gaze out on the same vista of stars and galaxies. Were we ever to contact them, perhaps the only common culture would stem from a shared interest in how our cosmic habitat emerged from an initial genesis event - the Big Bang.

We can in fact look back into the past, and see plain evidence of this evolution. Amazing long-exposure pictures taken with the Hubble Space Telescope show that the sky is densely covered with faint smudges of light, a billion times fainter than any star that can be seen with the unaided eye. These are really entire galaxies, each thousands of light years across, which appear so small and faint because they are so far away that their light set out 10 billion years ago. We see them as they were in the remote past - when all their stars were still young, and there were no planets, and presumably no life.

But what happened before galaxies formed? During the first few minutes of cosmic history, everything was squeezed hotter than the centre of the Sun, into a dense gas that was almost structureless - so simple that it can be described by just a few numbers: the "mix" of particles it contains, the way it is expanding, and the strengths of the basic forces. How did 12 billion years of cosmic evolution lead from such a simple "recipe" to our immensely complex cosmos? How - here on Earth and perhaps on other worlds - did atoms assemble themselves into creatures able to ponder their origins?

The answers will take us beyond the science we are familiar with - requiring new insights into the nature of space and time, and into the links between the cosmos and the microworld. Perhaps there are aliens, in orbits around distant suns, who already know the answers. But for us, these mysteries present a challenge for the new millennium - perhaps an unending quest.

Sir Martin Rees is the Astronomer Royal. His new book, 'Just Six Numbers' (Weidenfeld and Nicolson, £12.99), has just been published