British astronomers have found evidence of water in the atmosphere of a gas giant 64 light years from Earth, the first discovery of water in an extrasolar planet, they say.
Researchers at University College London (UCL) found that when the planet HD 189733b passes in front of its sun, "it absorbs starlight in a way that can only be explained by the presence of water vapour in its atmosphere," they said in a release Wednesday.
They used data from an orbital telescope to measure wavelengths in the infrared spectrum that demonstrated the presence of water in the planet, which orbits a star in the constellation Vulpecula the Fox.
The discovery is a step towards the "'holy grail" for the researchers seeking an Earth-like planet with water, said Giovanna Tinetti, lead researcher and a member of UCL's department of physics and astronomy.
"When it happens, that discovery will provide real evidence that planets outside our solar system might harbour life."
HD 189733b is about 15 per cent bigger than Jupiter and more than 30 times closer to its star than the Earth is to the sun.
That makes it extremely hot, so the water is not a liquid.
"It is not even steam; it is in the gaseous form, much like carbon dioxide, nitrogen or methane," said Bob Barber, also from UCL's department of physics and astronomy.
Barber and UCL colleague Prof. Jonathan Tennyson used data they generated by computer to identify water in the infrared spectrum of photos taken by NASA's Spitzer Earth-orbiting telescope. Their data is the most complete and accurate list of the spectral features due to hot water vapour, Barber said.
Their research produced a satisfactory model of the atmosphere of HD 189733b and has been used to identify water in other bodies in space, including comets, brown dwarfs, red giants and sun spots.
The research was published in the July 12 edition of Nature.
NASA says that the Spitzer Space Telescope has detected water vapour on HD 189733b, a massive exoplanet orbiting the dwarf star HD 189733, "trillions of miles" outside our solar system. The water vapours were found in the planet's atmosphere.

"We're thrilled to have identified clear signs of water on a planet that is trillions of miles away," said Giovanna Tinetti, a European Space Agency researcher and main author of NASA's study, at the Institute d’Astrophysique de Paris France.

The planet, termed a "wet Jupiter", takes approximately two days to orbit its sun and its surface temperatures are estimated to be at least 1,000 Kelvin (1,340 Fahrenheit/727 Centigrade) which, according to scientists, makes it unlikely that life would exist there. The presence of water was observed by using the telescope to examine the absorption spectra as the planet transited across its star.

"Finding water on this planet implies that other planets in the universe, possibly even rocky ones, could also have water," said co-author Sean Carey of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena.

Similar evidence for another hot gas giant, previously gathered using the Hubble telescope, was not widely accepted due to experimental noise and similar concerns.

HD 189733b is located 63 light-years away from Earth in the constellation Vulpecula.