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

Understand

We find that a theoretical fit to all the HD 209458b data at secondary eclipse requires that the dayside 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 $\mu$m is smaller than the corresponding radius in the optical.