Just a few years ago the search for alien life was the stuff of science fiction – the domain of writers, film makers and conspiracy theorists.

This weekend, however, in a house in Berkhamsted, Hertfordshire, two of Europe’s leading astronomers are finalising plans to turn that search into reality.

Glenn White and Malcolm Fridlund are the lead scientists for the Darwin mission, a space telescope so powerful that it would be able to spot distant planets, take photographs of them and even analyse their atmospheres for the telltale gases that indicate life.

For the past 10 years they and a team of astronomers around Europe have been perfecting the designs for Darwin. They are now collating that work into a formal proposal for the European Space Agency (ESA), which has already indicated it wants to adopt it as the centrepiece of Europe’s space research programme for the next decade.

“What we will be looking for is no less than life itself,” said White, professor of astronomy at the Open University. “The Darwin mission will be able to look at stars up to 100 light years away and see if the planets orbiting around them have evolved life.”

ESA’s plans for Darwin come at one of the most exciting times for planet hunters. In the past 12 years this specialised group of astronomers has found more than 220 planets orbiting other stars in the Milky Way – and its ability to detect ever-smaller extra-solar bodies is constantly improving.

Last week saw astronomers claim they had found potentially the smallest and most Earth-like planet yet, orbiting one of our closest neighbouring stars.

Gliese 581c was, it appeared, not just small and rocky, like the Earth. It also orbited its star (Gliese 581) in the so-called habitable zone, where the surface temperature could be just right to allow water to remain a liquid.

This week is likely to see at least one more such announcement, this time from the team overseeing Europe’s Corot satellite. It was launched just a few months ago but has already detected its first planet.

Another mission, Kepler, that is due to be launched by Nasa, the American space agency, next year, will accelerate that task, examining hundreds of stars for the minuscule fluctuations in light caused by planets passing across their faces.

“We are finding new planets faster than we ever expected,” said White. “What it suggests is that planets are very common and that many of them are likely to be like ours. It suggests that the chances of life having evolved on other planets are very high – but we won’t find it till we build something like Darwin.”

Such confidence shows the huge progress made in planet-hunting. Until the 1990s the only planets we knew of were those that orbited the sun in our solar system.

Then, in 1995, two astronomers, Michel Mayor and Didier Queloz, announced they had detected a new planet orbiting a star called 51 Pegasi.

Their discovery sent shock-waves through the scientific community. We were no longer alone. There were new worlds waiting to be discovered – and the methods used by Mayor and Queloz were simple and cheap enough for many astronomers to join the hunt.

Mayor’s detection method did not, however, study planets directly. Stars are so bright that relatively tiny objects such as planets are swamped by their light. Instead he measured the way stars “wobbled” under the gravitational pull of the planets orbiting around them.

By measuring such wobbles, astronomers can work out a great deal about a planet, including its probable size, distance from its star and even how warm it might be. This is how the Earth-like planet and its two companions orbiting the star Gliese 581 were detected.

Since 1995, however, astronomers have been working on a new method for detecting planets. The theory is that if a planet passes between you and a star, then the star dims slightly as it passes by.

So far this method has been used to detect only around a dozen planets, but it holds great potential. The Corot probe, for example, is already sending back data suggesting it has found dozens of new planets.

“The data is still being analysed but there are strong hints that there will be a lot more planets than we expected,” said Fridlund, a senior ESA scientist, who is involved in that mission as well as Darwin.

Another breakthrough in the hunt for planets came in 2004, when astronomers announced the first direct photograph of a planet orbiting another star.

There were all kinds of caveats: the star was a so-called brown dwarf, meaning it was too small to give off much light, and the planet was huge – around five times bigger than Jupiter.

What it showed, however, was that a new age of discovery was dawning. In particular, the astronomers had been able to pick out the telltale signatures of water and other chemicals in the light reflected from the planet’s atmosphere.

It meant we could not just find planets but we could also learn about the chemistry of their atmospheres. What’s more, we could also spot gases such as ozone, methane and oxygen that are the signatures of life.

The Darwin mission, planned for launch in 2015, will build on all these discoveries. It will comprise three space telescopes flying in close formation with a fourth spacecraft acting as a communications hub.

Each telescope will be small compared with those on Earth, but by acting together they will have the power of a far larger instrument. That power will be amplified still further by being in space – away from the distortions caused by the Earth’s atmosphere.

This means Darwin will be able not only to detect planets as small as the Earth but also to take photographs of them and then analyse their atmospheres for signs of life.

“This instrument will be more powerful than anything we have today,” said Fridlund. “This week we heard a lot about Gliese 581. It was a great discovery, but it took those researchers months to do that work. Darwin could have done the same detection work in eight minutes. If there was life there it would take 1.5 days to find it.”

Nasa is planning a mission very similar to Darwin, the Terrestrial Planet Finder. It, too, has been years in preparation but is now being threatened by Nasa’s new focus on returning to the moon and sending humans to Mars, missions decreed by President George Bush.

This means Nasa’s planet hunters are likely to drop their own project and join forces with Europe in building the Darwin probes, making it a truly global mission.

Once started, however, the search for alien life may prove never-ending. Sir Martin Rees, the astronomer royal, has pointed out that the universe contains at least 100 billion galaxies, each with 100 billion stars. It means there are more stars than all the grains of sand on every beach and in every desert on Earth.

For White and Fridlund, however, that is a source of optimism, not despair. They point out that if humans have been able to find so many planets so quickly around the relatively few stars close enough to measure, then how many more must there be waiting to be found?

“Statistically there could be billions of Earth-like planets out there,” said White. “I have ultimate faith that in the next few years we will find life elsewhere in the universe.”

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