The "hot Jupiter" planet's surface temperatures exceed 900C.
Writing in the journal Nature, the scientists say their discovery may help
find planets that can support life.
In a separate study, the US space agency (Nasa) said that it had found carbon
dioxide (CO2) in the atmosphere of the same planet.
Molten core
The planet known as HD 189733b is classed as a hot Jupiter due to its fiery
molten centre and heavily gaseous atmosphere, which mimics the atmosphere of
Jupiter, the gas giant in our own solar system.
The generation of heat by the planet's core provides the key to why
scientists have been able to identify water vapour in its atmosphere.
Gases in the planet's atmosphere modify the wavelengths of heat radiation
coming from the planet's hot surface. These wavelengths can be detected by space
telescopes such as Hubble or the Sun-orbiting Spitzer telescope used in this
study.
The type of gas 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 by studying its orbit.
"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 taking
measurements over several orbiting cycles and deducting the radiation produced
during the time when they couldn't see the planet - when it was behind its star
- they were able to see how much radiation the planet emitted on its own.
"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.
Carbon Dioxide
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, the star covers perhaps half the
planet, 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, Nasa says the Hubble space telescope has detected
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.
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