On the Inference of Thermal Inversions in Hot Jupiter Atmospheres

TL;DR

This study investigates the degeneracies between thermal inversions and molecular abundances in hot Jupiter atmospheres, emphasizing the need for thorough model exploration to accurately infer thermal inversions from limited observational data.

Contribution

It demonstrates that extensive parameter space exploration is crucial for correctly identifying thermal inversions, showing that current data can fit models both with and without inversions for some planets.

Findings

Models with and without thermal inversions fit TrES-4 and TrES-2 data equally well.

HD 209458b and HAT-P-7b are better fit with thermal inversions.

No correlation found between irradiation levels and thermal inversions.

Abstract

Several studies in the recent past have inferred the existence of thermal inversions in some transiting hot Jupiter atmospheres. Given the limited data available, the inference of a thermal inversion depends critically on the chemical composition assumed for the atmosphere. In this study, we explore the degeneracies between thermal inversions and molecular abundances in four highly irradiated hot Jupiter atmospheres, day-side observations of which were previously reported to be consistent with thermal inversions based on Spitzer photometry. The four systems are: HD 209458b, HAT-P-7b, TrES-4, and TrES-2. For each system, we explore the model parameter space with ~ 10^6 models using a Markov chain Monte Carlo routine. Our results primarily suggest that a thorough exploration of the model parameter space is necessary to identify thermal inversions in hot Jupiter atmospheres. We find that existing observations of TrES-4 and TrES-2 can both be fit very precisely with models with and without thermal inversions, and with a wide range in chemical composition. On the other hand, observations of HD 209458b and HAT-P-7b are better fit with thermal inversions than without, as has been reported before. We do not see a correlation between irradiation levels and thermal inversions, given current data. Before JWST becomes available, near-IR observations from ground and with HST, along with existing Spitzer observations, can potentially resolve thermal inversions in some systems.