Water Vapor in an Exoplanet’s Atmosphere

by Paul Gilster on April 12, 2007

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.

Hans Bausewein April 14, 2007 at 5:54

David Charbonneau about the water find:

“That’s a possibility but not yet certain, says astrophysicist David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, whose data and technique formed the basis for Barman’s conclusions.”


ljk May 18, 2007 at 15:32

Analysis of Spitzer Mid Infrared Spectra of Irradiated Planets: Evidence for Water Vapor?

Authors: Jonathan J. Fortney, Mark S. Marley

(Submitted on 16 May 2007)

Abstract: Published mid infrared spectra of transiting planets HD 209458b and HD 189733b, obtained during secondary eclipse by the InfraRed Spectrograph (IRS) aboard the Spitzer Space Telescope, are predominantly featureless. In particular these flux ratio spectra do not exhibit an expected feature arising from water vapor absorption short-ward of 10 microns. Here we suggest that the spectral data for HD 189733b are inconsistent with 8 micron-photometry obtained with Spitzer’s InfraRed Array Camera (IRAC), perhaps an indication of problems with the IRS spectra from 7.5-10 microns. The IRAC data, along with previously published secondary eclipse photometry for HD 189733b, are in excellent agreement with a one-dimensional model of HD 189733b that clearly predicts water absorption. We are not able to draw firm conclusions regarding the IRS data for HD 209458b, but spectra predicted by 1D and 3D atmospheres models fit the data adequately, without adjustment of the water abundance or reliance on cloud opacity. We argue that the generally good agreement between model spectra and IRS spectra of brown dwarfs with atmospheric temperatures similar to these highly irradiated planets support the veracity of the modeling procedure.


Submitted to ApJL


Astrophysics (astro-ph)

Cite as:

arXiv:0705.2457v1 [astro-ph]

Submission history

From: Jonathan J. Fortney [view email]

[v1] Wed, 16 May 2007 23:05:19 GMT (43kb)


ljk May 24, 2007 at 9:48

ASTRONOMERS SEEK LIFE AT END OF RAINBOW (Space & Astronomy News, 21/5/07)

Rainbows may be the key to identifying habitable planets around nearby stars,
according to a researcher who says light scattering could indicate the presence
of liquid water.


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