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

Detection of molecules using infrared spectroscopy probes the conditions and compositions of exoplanet atmospheres. Water (H₂ O), 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 μm spectral region. Here we report ground-based observations of the dayside emission spectrum for HD 189733b between 2.0-2.4 μm and 3.1-4.1 μm, 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 μm is unexpected and difficult to explain with models that assume local thermodynamic equilibrium (LTE) conditions at the 1 bar to 1 ×-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.