New Analysis Indicates No Thermal Inversion in the Atmosphere of HD 209458b

TL;DR

A comprehensive re-analysis of Spitzer data shows that HD 209458b's atmosphere does not exhibit a thermal inversion, challenging previous assumptions about this prototypical hot Jupiter.

Contribution

This study provides a new, self-consistent analysis of all available data, demonstrating the absence of thermal inversion in HD 209458b's atmosphere.

Findings

No evidence of thermal inversion in HD 209458b

Secondary-eclipse depths consistent with a monotonic temperature profile

Data well-fitted by a non-inverted atmospheric model

Abstract

An important focus of exoplanet research is the determination of the atmospheric temperature structure of strongly irradiated gas giant planets, or hot Jupiters. HD 209458b is the prototypical exoplanet for atmospheric thermal inversions, but this assertion does not take into account recently obtained data or newer data reduction techniques. We re-examine this claim by investigating all publicly available Spitzer Space Telescope secondary-eclipse photometric data of HD 209458b and performing a self-consistent analysis. We employ data reduction techniques that minimize stellar centroid variations, apply sophisticated models to known Spitzer systematics, and account for time-correlated noise in the data. We derive new secondary-eclipse depths of 0.119 +/- 0.007%, 0.123 +/- 0.006%, 0.134 +/- 0.035%, and 0.215 +/- 0.008% in the 3.6, 4.5, 5.8, and 8.0 micron bandpasses, respectively. We feed these results into a Bayesian atmospheric retrieval analysis and determine that it is unnecessary to invoke a thermal inversion to explain our secondary-eclipse depths. The data are well-fitted by a temperature model that decreases monotonically between pressure levels of 1 and 0.01 bars. We conclude that there is no evidence for a thermal inversion in the atmosphere of HD 209458b.