A ground-based near-infrared emission spectrum of the exoplanet HD 189733b

TL;DR

This study presents ground-based near-infrared emission spectra of exoplanet HD 189733b, revealing unexpected features likely due to non-LTE methane emission, challenging existing models and emphasizing the importance of ground-based observations.

Contribution

First ground-based detection of HD 189733b's emission spectrum in the 2.0-4.1 micron range, identifying non-LTE methane emission features not explained by current models.

Findings

Detected a bright emission feature at 3.25 microns.

Results agree with space-based measurements where overlapping.

Identified non-LTE methane emission as a likely cause of the unexpected feature.

Abstract

Detection of molecules using infrared spectroscopy probes the conditions and compositions of exoplanet atmospheres. Water (H2O), methane (CH4), carbon dioxide (CO2), and carbon monoxide (CO) have been detected in two hot Jupiters. These previous results relied on space-based telescopes that do not provide spectroscopic capability in the 2.4 - 5.2 micron spectral region. Here we report ground-based observations of the dayside emission spectrum for HD 189733b between 2.0-2.4 micron and 3.1-4.1 micron, where we find a bright emission feature. Where overlap with space-based instruments exists, our results are in excellent agreement with previous measurements. A feature at ~3.25 micron is unexpected and difficult to explain with models that assume local thermodynamic equilibrium (LTE) conditions at the 1 bar to 1 x 10-6 bar pressures typically sampled by infrared measurements. The most likely explanation for this feature is that it arises from non-LTE emission from CH4, similar to what is seen in the atmospheres of planets in our own Solar System. These results suggest that non-LTE effects may need to be considered when interpreting measurements of strongly irradiated exoplanets.