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Published October 20, 2007 | Published
Journal Article Open

Theoretical Spectral Models of the Planet HD 209458b with a Thermal Inversion and Water Emission Bands


We find that a theoretical fit to all the HD 209458b data at secondary eclipse requires that the day-side atmosphere of HD 209458b have a thermal inversion and a stratosphere. This inversion is caused by the capture of optical stellar flux by an absorber of uncertain origin that resides at altitude. One consequence of stratospheric heating and temperature inversion is the flipping of water absorption features into emission features from the near- to the mid-infrared, and we see evidence of such a water emission feature in the recent HD 209458b IRAC data of Knutson et al. In addition, an upper-atmosphere optical absorber may help explain both the weaker-than-expected Na D feature seen in transit and the fact that the transit radius at 24 μm is smaller than the corresponding radius in the optical. Moreover, it may be a factor in why HD 209458b's optical transit radius is as large as it is. We speculate on the nature of this absorber and the planets whose atmospheres may, or may not, be affected by its presence.

Additional Information

© 2007 American Astronomical Society. Received 2007 August 1; accepted 2007 August 28; published 2007 September 26. We thank Drake Deming, Bill Hubbard, Maki Hattori, Mike Cushing, and Drew Milsom for helpful discussions. This study was supported in part by NASA grants NNG04GL22G and NNX07AG80G and through the NASA Astrobiology Institute under cooperative agreement CAN-02-OSS-02 issued through the Office of Space Science.

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