An artist's impression of the Jupiter-like HD209458b. Gases are lost from the atmosphere of the planet the as it orbits a star in the constellation Pegasus.

Credit: ESA and Alfred Vidal-Madjar

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WASHINGTON: The first evidence of water in the atmosphere of a planet outside our own Solar System has been discovered by U.S. astronomers.

The find suggests that many planets orbiting other stars may also harbour water, a key building block of life.

To make the discovery, the astronomers drew on data and measurements taken by the Hubble Space Telescope, and applied new theoretical models to them to demonstrate the likelihood of water absorption in the atmosphere of extra-solar planet HD209458b.

Learning about how water is distributed on other planets may help us better understand whether conditions for life are possible.

For years scientists have anticipated that 'exoplanets' would show signs of water in the atmosphere - even those that orbit much closer to central stars than our solar system's Mercury sits from the Sun. But that proximity has made detecting water difficult, according to astronomers.

HD209458b, which sits 150 light years from Earth in the constellation Pegasus, was the first transiting exoplanet ever found - that is, the first discovered that passes in front of a much larger body, from the vantage point of the Earth. It is also the first exoplanet known to have an atmosphere.

The Jupiter-like gas giant orbits its star every three and a half days, and each time it goes by, scientists can assess the contents of its atmosphere by how it absorbs light from the star.

Heather Knutson of Harvard University in Boston, Massachusetts collected Hubble Telescope data on HD209458b and applied to it new theoretical models devised by Travis Barman of the Lowell Observatory in Flagstaff, Arizona. The pair subsequently identified water absorption in the planet's atmosphere.

Atmospheric water absorption in such an exoplanet renders it larger in appearance across one part of the infrared spectrum, compared to wavelengths in the visible spectrum said the astronomers in a written summary of their research – which they will publish in an upcoming Astrophysical Journal.

"It is encouraging that theoretical predictions of water in extrasolar planets seem to agree reasonably well with observations," said Barman. "We now know that water vapor exists in the atmosphere of one extrasolar planet and [now] there is good reason to believe that other extrasolar planets contain water vapor."

Another group of astronomers had announced earlier this year that they could not detect water on HD209458b or another similar extrasolar planet.