Another discovery thanks to transits. The atmosphere of the exoplanet HD 209458b has been found to contain water vapor. And while that’s not unexpected, the effectiveness of the transit method in making the find underlines how significant are the occasions when a planet passes in front of its star as seen from Earth. Studying the infrared spectrum, as Travis Barman did at Lowell Observatory, shows the apparent signature of water vapor absorption when compared to the visible spectrum.
But don’t expect an ocean world here. The planet involved orbits its star every three and a half days; HD 209458b is, in fact, a ‘hot Jupiter,’ its upper atmosphere heated to temperatures as high as 10,000 degrees K. The planet is doubtless losing thousands of tons of material every second as it vents gases into the incendiary environment so near its primary.
Nonetheless, finding water vapor does provide confirmation of theories that suggest almost all extrasolar planets have water vapor in their atmospheres. “We know that water vapor exists in the atmospheres of one extrasolar planet,” says Barman, “and there is good reason to believe that other extrasolar planets contain water vapor.”
Barman compared his theoretical models with visible and infrared Hubble Space Telescope data on the star collected by Harvard graduate student Heather Knutson. The paper on the work is Barman, “Identification of Absorption Features in an Extrasolar Planet Atmosphere,” accepted for publication in The Astrophysical Journal (preprint available). Also see Dennis Overbye’s story in the New York Times, which introduces questions regarding some of Barman’s data in connection with earlier observations.