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For the first time ever, water has been conclusively identified in the atmosphere of a planet outside our solar system.

The presence of water on another planet is a groundbreaking discovery, and has fuelled hopes that one day we will find life somewhere out there. Water is widely regarded as being an essential ingredient for life. Certainly on Earth, wherever there is water and energy, there lurks something living, however slimy or weird.

Unfortunately for seekers of little green men to whom we could come in peace, the planet would be very unfamiliar to anyone who evolved in this neighbourhood. HD 189733b, located 63 light-years away, in the constellation Vulpecula, is a gas giant slightly larger than Jupiter, and orbits its star once every 2.2 days.

The researchers, who made the discovery using NASA's Spitzer space telescope, say the discovery lays the ground work for detecting water, and possibly life, on more familiar, rocky worlds like Earth.

Dr Giovanna Tinetti, an ESA fellow from the Institute d'Astrophysique de Paris and UCL said: "The 'holy grail' for today's planet hunters is to find an Earth-like planet that also has water in its atmosphere. That discovery, when it happens, will provide real evidence that planets outside of our Solar System might harbour life. So the discovery that water exists on an extra-solar gas giant is a vital milestone along that road of discovery."

The watery gas giant is a transiting planet - it passes directly between us and its star as it orbits. This gives astronomers an opportunity to study the spectrum of light that passes through the planet's atmosphere from its parent star.

The researchers, led by Dr Tinetti, found that the planet absorbs the starlight in a way that can only be explained if it has water vapour in its atmosphere.

The starlight that had passed through the planet's atmosphere dimmed at two infrared bands: 3.6 and 5.8 micrometres. If the planet was rocky world with no atmosphere, both bands (and a third at 8 micrometres) would respond the same way. But in this case, the atmosphere absorbed less radiation at 3.6 micrometres than at either 5.8 or 8 micrometres.

Water is the only molecule that can explain this behaviour, the researchers say.

As for finding water on smaller planets, first we have to find the planets. Today's telescopes are not quite sensitive enough to detect the gravitational wobble exerted by smaller, rocky worlds that will be so much more interesting to us. But when the James Webb Telescope launches in 2011, some smaller planets might well become targets for this kind of analysis. ®

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