Sixty-four light years away, in the constellation of Vulpeca the Fox, the water of life has at last been sighted. Using Nasa's infrared Spitzer space telescope, astronomers at the European Space Agency have discovered a remarkable planet. HD 189733b absorbs starlight in a way that can be explained only by the presence of water vapour or steam in its atmosphere.

This is the first time that astronomers have been able to confirm that water is present on an extra-solar planet. The discovery increases the chances of life being found among the stars - but probably not on this exotic world.

HD 189733b is a gas giant, which orbits so close to its parent star that its surface reaches a scorching 1700F. Winds rage around it at up to 6000mph - about 30 times faster than the Earth's jet streams.

"Although HD 189733b is far from being habitable, and actually provides a rather hostile environment, our discovery shows that water might be more common out there than previously thought," said Dr Giovanna Tinetti, of the European Space Agency, who led the study, published today in Nature.

"The holy grail for today's planet hunters is to find an Earth-like planet that also has water in its atmosphere.

"When it happens, that discovery will provide real evidence that planets outside our solar system might harbour life. Finding the existence of water on an extra-solar gas giant is a vital milestone along that road of discovery."

More than 200 extra-solar planets have been discovered outside our solar system, orbiting stars close to our own Sun. But finding one that harbours water is no easy feat.

Water is liquid only within a narrow window of temperatures and pressures. For life to flourish, a planet has to orbit its sun at just the right distance - an area known as the Goldilocks zone.

So it was something of a fairy tale this April when Gliese 581c, the first Goldilocks planet, was discovered circling a dim star in Libra. With a pleasant surface temperature of between 0 and 40C, Gliese 581c was hailed as the best hope yet for alien hunters. "On the treasure map of the universe, one would be tempted to mark this planet with an X," said Xavier Delfosse of Grenoble University, one of the planet's discoverers. But, alas, it now seems that Gliese 581c is no utopia.

A new study, assuming Earthlike characteristics for its geology and atmosphere, has concluded that the planet is likely to be a stifling greenhouse, utterly unsuitable for life. "It's just too hot," said Dr Manfred Cuntz, an astronomer at the University of Texas at Arlington. "I would not recommend mankind to move to that planet right now."

In a exciting twist, however, Dr Cuntz and his team said that another planet in the system - Gliese 581d - had emerged from the calculations as a Goldilocks candidate.

The planet orbits further away from the sun - 23 million miles, a distance that would usually make it too cold for liquid water. But Dr Cuntz believes that the same greenhouse effect that would scorch Gliese 581c would warm the larger outer one and make it habitable.

"Despite the adverse conditions of this planet, at least some primitive forms of life may be able to exist on its surface", he said, in a paper submitted to the journal Astronomy and Astrophysics.

Other astronomers, however, describe the new paper as speculative.

The main lesson, according to Sara Seager, a planetary theorist at the Massachusetts Institute of Technology, is that finding Goldilocks planets is much more complicated than measuring the distance of planetary orbits, and that the properties of the planet come into play. "Astronomers are going to realise just how complicated the idea of habitability is," she said. "The whole of astronomy is built on not knowing anything."