WASHINGTON, July 11 (Xinhua) -- An international team of astronomers report the first conclusive discovery of the presence of water vapor in the atmosphere of a planet beyond our Solar System.

    The findings appear in the 12 July issue of journal Nature. Giovanna Tinetti, European Space Agency fellow and colleagues from around the world, used data from NASA's Spitzer Space Telescope. They targeted planet HD 189733b, 63 light-years away, in the constellation Vulpecula.

    The planet was discovered in 2005 as it dimmed the light of its parent star by some three percent when transiting in front of it. Using Spitzer, Tinetti and the team observed the star, which is slightly fainter than the Sun. They watched its starlight dim at two infrared bands (3.6 and 5.8 micrometers).

    Had the planet been a rocky body devoid of atmosphere, both these bands and a third one (8 micrometers), recently measured by a team at Harvard, would have shown the same behavior.

    Instead, as the planet's tenuous outer atmosphere slipped across the face of the star, the starlight absorbed showed a different, distinctive pattern. The atmosphere absorbed less infrared radiation at 3.6 micrometers than at the other two wavelengths.

    "Water is the only molecule that can explain that behavior," says Tinetti.

    The presence of water vapor does not necessarily make it a good candidate in the search for planets that harbor life. "This is a far from habitable world," she adds.

    Instead of a rocky world like Earth, HD 189733b is large, about1.15 times the mass of Jupiter. Located just 4.5 million kilometers from its star, it orbits it in 2.2 days. In comparison, Earth is 150 million kilometers from the Sun; even Mercury, the innermost planet, is 70 million kilometers away.

    Astronomers classify such worlds as "hot jupiters." HD 189733b's atmospheric temperature is about 1000 Kelvin (a little more than700C) or higher, implying that the significant amounts of water vapor in the atmosphere cannot condense to fall as rain or form clouds. The temperature would have to be about five times lower to form clouds of water vapor or rain.

    Although, being a gas giant, the planet is an unlikely candidate in the search for life, these results increase hopes for the detection of water on other rocky planets, which astronomers hope to discover in the near future.