The 3.6-8.0 Micron Broadband Emission Spectrum of HD 209458b: Evidence for an Atmospheric Temperature Inversion

TL;DR

This study measures the infrared emission spectrum of exoplanet HD 209458b, providing the first evidence of a temperature inversion layer in a hot Jupiter's atmosphere, challenging standard atmospheric models.

Contribution

It presents the first observational evidence of an atmospheric temperature inversion in a hot Jupiter using multi-band infrared eclipse data.

Findings

Detection of emission features indicating a temperature inversion

Eclipse depths differ from standard atmosphere model predictions

Evidence of water emission in specific infrared bands

Abstract

We estimate the strength of the bandpass-integrated thermal emission from the extrasolar planet HD 209458b at 3.6, 4.5, 5.8, and 8.0 microns using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We observe a single secondary eclipse simultaneously in all four bandpasses and find relative eclipse depths of 0.00094 +/- 0.00009, 0.00213 +/- 0.00015, 0.00301 +/- 0.00043, and 0.00240 +/- 0.00026, respectively. These eclipse depths reveal that the shape of the inferred emission spectrum for the planet differs significantly from the predictions of standard atmosphere models; instead the most plausible explanation would require the presence of an inversion layer high in the atmosphere leading to significant water emission in the 4.5 and 5.8 micron bandpasses. This is the first clear indication of such a temperature inversion in the atmosphere of a hot Jupiter, as previous observations of other planets appeared to be in reasonably good agreement with the predictions of models without such an inversion layer.