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

TL;DR

This paper models HD 209458b's atmosphere, revealing a thermal inversion and water emission features caused by an unknown optical absorber, explaining various observational data and suggesting new atmospheric dynamics insights.

Contribution

It introduces a theoretical spectral model incorporating a thermal inversion and water emission, explaining multiple observational features of HD 209458b.

Findings

Evidence of water emission features in IRAC data.

A thermal inversion caused by an unknown optical absorber.

Explanation for the planet's transit radius variations.

Abstract

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. 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.