Last week NASA announced the discovery of vast quantities of water ice under the Martian surface, and the dusty desert model of the Red Planet is now undergoing a serious re-think. No one is claiming that there is life there - well, not yet, anyway - but the presence of so much water raises the possibility that life could have evolved there in the past, and that we could inhabit it in the future.

But the fact remains that for all intents and purposes, right now Mars is as dead as vaudeville. There may be fossils of microorganisms, and this in itself would be a stupendous discovery, meaning that life had evolved independently in two separate environments, but almost certainly there will be no worms or lichens or slime, let alone ancient civilisations or even a decent pub.

We may yet find life on Europa, one of the moons of Jupiter thought to be hiding a massive ocean under its icy crust. But ironically, the same week that NASA discovered the ice on Mars, they also found that the ice on Europa is much thicker than first thought - up to 20 kilometres - and in contrast to the Mars find, this is a "bad news" ice discovery. Plans to bore through the ice to look for Europan water bugs suddenly look like hard work.

The rest of the solar system is now considered a complete write-off when it comes to finding life. Venus and Mercury are too hot, and just about everywhere else is either too cold or impossibly hostile. The giant gas planets like Jupiter are simply too alien to support aliens.

But there are other solar systems. In the last ten years, using sophisticated techniques that measure the effect of orbiting bodies on the movements of distant stars, astronomers have discovered nearly 100 planets, and it's still early days. All of the planets discovered so far have been the size of Jupiter or bigger, and none of them are thought to be likely candidates for anything life-like. But this doesn't mean that earth-like planets aren't out there too.

Dr. Charles Lineweaver of the University of New South Wales has spent the last several years estimating the probability of "earths", or terrestrial planets, orbiting stars in our galaxy, and has come to some fascinating conclusions. In essence, he's convinced that the Universe is teeming with planets very similar to ours, and that it is almost inevitable that life has arisen on many of them.

"By analysing the metallic content of various star systems we can estimate the likelihood of there being terrestrial planets, as well as estimating their ages," he says.

"We believe now that around three quarters of all earthlike planets are older than the earth, by an average of 1.8 billion years.

"If life forms readily on earthlike planets - as suggested by the rapid appearance of life on earth - this gives an age distribution for life on such planets and a rare clue about how we might compare to other life which may inhabit the universe.

"It depends how you define a terrestrial planet, of course. It's like a human being. Humans have lots of things in common, but every human is unique. It's the same with planets. My definition of a terrestrial planet that could be home to life is 'a rocky world with liquid surface water'.

"But there are variables. A large moon, the presence of Jupiter-sized planets in the system, the type of star the planet orbits. All these things affect what these worlds would be really like."

Barrie Jones and Nigel Sleep at the Open University in the UK have also been looking beyond the giant worlds that orbit other stars and trying to work out the best places to start searching for terrestrial planets. They too are optimistic.

"Although we don't yet have the capability to detect 'tiddlers' like the earth, we can establish theoretically which of the exoplanetary systems are most likely to have an 'Earth'" says Jones.

The giant planets are the bugbear here. Most of the worlds found so far orbit very close to their stars, Jones points out, and his team have been devising "computer models of known exosystems to work out how long a terrestrial planet would survive before being ejected by gravitational forces.

"There could be at least a billon 'Earths" in the Milky Way," he says "and lots more if we find systems more like ours, with their giant planets well away from the habitable zones."

Jones and Sleep think they may have found a good candidate, the star 47 Ursa Majoris. Its two known gas giants are closer to it than Jupiter is to the sun, but far enough out to allow an "earth" to hold a stable orbit in what's known in the trade as "the Goldilocks Zone'. Of course, this is where the temperature is not too hot, not too cold, but just right. There may well be an Earth in there somewhere. We just don't know.

The fact that most of these solar systems have huge planets orbiting close to their stars doesn't mean that this is the norm and that we are somehow special. It's just that massive planets close to a star are the easiest to detect because they have the greatest effect on the star's perceived motion. It's unlikely that we could detect our own solar system, even Jupiter, from a similar distance with our current methods. Detecting Earth from interstellar space would be impossible, just as it's been impossible to detect exosolar Earths from here.

Until now we simply haven't had equipment sensitive enough to find them, but this is all about to change. In 2006 NASA will launch Kepler, an orbiting telescope that it is hoped will give us the first concrete evidence of earth-like planets in deep space.

The idea is not to measure the gravitational effects of the planets on their parent stars. This would in any case be so minuscule as to be undetectable. Rather, the Kepler telescope will survey 100,000 stars and wait for a terrestrial planet to pass across the face of the star, in much the same way as Captain Cook observed the transit of Venus in 1770. These measurements allowed astronomers to calculate the size and the orbit of Venus, and applying the same principal to extra-solar planets would yield the same information. At long last we'd know whether we had a look-alike in the Goldilocks Zone.

The difference between the expeditions of HMS Endeavour and Kepler is that Venus was easy to see. It will be impossible for Kepler to actually see these planets, but it will just be able to detect the slight dimming of the stars as they pass in front of them.

This is no small task. A planet the size of the Earth would dim the light of its star by less than 1/10,000th, and what's more, Kepler has to be lucky enough to find solar systems aligned precisely edge-on for a transit to be observed at all, and the odds of that are less than one percent.

Nonetheless, if astronomers like Charles Lineweaver and Barrie Jones have done their sums right, it seems very likely that when the Kepler mission ends in 2010, we should have found a swag of these wet rocks spinning around sun-like stars.

Establishing whether the natives are friendly, or even exist, might take a little longer. But the one terrestrial planet in the Goldilocks Zone discovered so far is teeming with life.

The question is - is just one planet a statistically significant sample?

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