The chance of finding life beyond our Solar System took a leap in 1995 when Michel Mayor and Didier Queloz at the Geneva Observatory discovered the first planet orbiting another star. Since then, ground-based telescopes have detected no fewer than 74 planets orbiting 60 stars - and the number is rising almost monthly.

Most of these planets, however, are large (at least the size of Jupiter) and orbit too close to their stars for life's comfort. Earth-like planets, capable of supporting life as we know it, will be much smaller and orbit their stars within 'habitable zones', that is at such a distance and in such a way that liquid water can exist on their surfaces most of the time.

"About 1000 stars are being searched for planets at the moment and it seems that about 5 per cent of them have closely orbiting Jupiters. The reason we haven't detected small planets is because our methods are not sensitive enough," says Alan Penny, a member of the Darwin study team from the Rutherford Appleton Laboratory, United Kingdom. "If you think of the extra-solar planets as the occupants of a zoo, we've only looked inside one cage so far - the one containing large planets near to their star," he adds.

Increases in the sensitivity of ground-based telescopes are expected to take the size of detectable extra-solar planets down to one sixth of a Jupiter, or 50 Earths in the next ten years. But "space is best", says Penny and plans are afoot at ESA to launch three spacecraft to look for Earth-like planets and the signs of life on them.

First will be Eddington with a possible launch date of 2008, which will search 500 000 stars for orbiting Earth-like planets. Next will be Gaia which will watch for stars wobbling along their orbits because they are being tugged by planets. Finally, Darwin will employ a sophisticated technique called nulling interferometry to detect Earth-like planets directly and determine the composition of their atmospheres through spectral analysis.

"If we see oxygen or ozone, we'll know that something is pumping it out continuously, as oxygen is very reactive and doesn't stay around long. People have wracked their brains to think of methods of getting a lot of oxygen into the atmosphere that have nothing to do with life - and they can't think of any. So in all probability, if we see oxygen or ozone, it will mean life," says Penny.

Darwin, however, is a complex and challenging mission technologically. "Recent studies by industry have shown Darwin to be feasible within the next decade," says Malcolm Fridlund, Darwin study scientist at ESA. "Nevertheless, a very large amount of technological preparatory work is needed which will be carried out in pre-cursor missions over the next few years."

Whether it's looking for life close by in our Solar System or far away in distant galaxies, ESA is forging forward with exciting and ambitious exploration plans to enable European scientists and engineers come closer to answering the question: "Is there life out there?"