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Hubble spots water on distant planet

12 Apr 2007

Optical observations have helped astronomers to discover what they believe to be the first extrasolar water-containing planet.

Lowell Observatory astronomer Travis Barman has found strong evidence for water absorption in the atmosphere of an extrasolar planet (arXiv: 0704.1114v1 [astro-ph] 9 April 2007).

"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," reported Barman in a press release issued by Lowell Observatory.

Water vapor has been expected to be present in the atmospheres of nearly all of the known extrasolar planets. Detecting water, however, is difficult because of the planets proximity to their parent stars.

Detection was possible here thanks to the orbital path of the planet HD209458b passing directly in front of its star every three and a half days. As a planet passes in front of a star, 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.

Comparison of visible and infrared data from the Hubble Space Telescope with new theoretical models developed by Barman has led to the identification of water absorption in HD209458b which is 150 light years from Earth.

The data was collected by the Space Telescope Imaging Spectrograph acting like a prism separating light into its component colors. The spectrograph is able to provide a wavelength "fingerprint" of the object being observed.

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