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Planetary News: Extrasolar Planets (2007)

Water Identified in Extrasolar Planet Atmosphere

 

April 11, 2007
Artist's Rendering of Transit of HD209458
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Artist's Rendering of Transit of HD209458
In 1999, astronomers witnessed for the first time a distant planet passing in front of its star. The passage of a planet between a star and the Earth is called a "transit." If such a dimming is detected at regular intervals and lasts a fixed length of time, then it is very probable that a planet is orbiting the star and passing in front of it once every orbital period. Credit: Lynette Cook

For the first time, water has been identified in the atmosphere of an extrasolar planet. Through a combination of previously published Hubble Space Telescope measurements and new theoretical models, Lowell Observatory astronomer Travis Barman has found strong evidence for water absorption in the atmosphere of transiting planet HD209458b.

"We now know that water vapor exists in the atmosphere of one extrasolar planet, and there is good reason to believe that other extrasolar planets contain water vapor," said Barman.

Water vapor (or steam) has been expected to be present in the atmospheres of nearly all of the known extrasolar planets, even those that orbit closer to their parent star than Mercury is to our Sun. For the majority of extrasolar planets, their close proximity to their parent star has made detecting water and other compounds difficult.

As seen from Earth, HD209458b passes directly in front of its star every three and half days, and when it does, its atmosphere blocks a different amount of the starlight at different wavelengths. In particular, absorption by water in the atmosphere of a giant planet makes the planet appear larger across a specific part of the infrared spectrum compared to wavelengths in the visible spectrum.

An analysis of visible and infrared Hubble data carried out last year by Harvard student Heather Knutson made possible a direct comparison to new theoretical models developed by Barman at Lowell Observatory. This ultimately led to the identification of water absorption in a planet 150 light years from Earth.

"It is encouraging that theoretical predictions of water in extrasolar planets seem to agree reasonably well with observations," said Barman.