By Julian Siddle
Science Reporter, BBC News
Scientists used an eclipse method to make the detection
Scientists say they have found evidence for water vapour in the atmosphere of a planet 63 light-years from Earth.
The "hot Jupiter" planet's surface temperatures exceed 900C.
Writing in the journal Nature, the researchers say their discovery may help find planets that can support life.
In a separate study, another group of scientists said that it had detected carbon dioxide (CO2) in the atmosphere of the same planet.
The planet known as HD 189733b is classed as a hot Jupiter because it is a gas giant orbiting close to its parent star.
The gases present in the planet's atmosphere can be determined by looking at the spread of infrared radiation reaching the telescope, each gas producing a different wavelength.
Dr Drake Deming from Nasa's Goddard Space Flight Center, Maryland, US, has looked for signs of water on similar gas giants in the past. He says water vapour in the atmosphere leaves an unmistakeable signal.
"It produces a unique fingerprint, water vapour modulates the shape of the radiation in a very characteristic way," he said.
As the planet is so far away it is hard to determine how much of the radiation detected by the telescope comes from this gas giant and how much from the star it orbits.
The scientists solved this problem using an eclipse technique, looking for differences in infrared light detected when the planet passed behind and in front of its parent star. The observations were made with the Spitzer space telescope.
"There is a time when we know the planet is not visible, so we know the light comes only from the star," says Dr Carl Grillmair from the Spitzer Science Center at the California Institute of Technology, who led the research.
They found HD 1897733b goes round its star every 2.2 days. By measuring the dip in infrared light that occurs when the planet disappears, Spitzer can learn how much light is coming solely from the planet.
"The key to these measurements is the eclipse geometry, we have a unique moment in which to observe the star in isolation," said Dr Deming.
The scientists were puzzled by earlier observations of HD 189733b and similar gas giants. They expected to see water vapour, but the telescopes did not detect any.
"We concluded there was no water a couple of years ago, the theoreticians were upset, they'd predicted it would be there. We didn't understand it. We looked much harder we watched it for over 120 hours, and sure enough there was the signature matching brilliantly with the models," said Dr Grillmair.
He suggests the planet's proximity to the star means its atmosphere is constantly changing.
"With planets this close to their star... you're going to get enormous heat loads that create storms, perhaps clouds one year and none the next - this thing is changing right before our eyes" said Dr Grillmair.
The scientists suggest high clouds created by the storms may have hidden the water vapour in the earlier observations. They are confident that the latest findings are correct.
"What's new about this is it's unequivocal," says Dr Deming.
In a separate development, researchers used the Hubble space telescope to detect carbon dioxide in HD189733's atmosphere.
Although the agency is keen to stress the planet is far too hot to support life, it says the finding represents an important proof of concept, showing that it is possible to detect CO2 in the atmospheres of distant planets orbiting other stars, and that the same method could be used to look at planets which might support life.
"The very fact we are able to detect it and estimate its abundance is significant for the long-term effort of characterising planets to find out what they are made of and if they could be a possible host for life," said Mark Swain, a research scientist at Nasa's Jet Propulsion Laboratory in Pasadena, California, who analysed the Hubble images.