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 (H(2)O), methane (CH(4)), carbon dioxide (CO(2)), and carbon monoxide (CO) have been detected(1-5) in two hot Jupiters. These previous results relied on space-based telescopes that do not provide spectroscopic capability in the 2.4-5.2 mu m spectral region. Here we report ground-based observations of the dayside emission spectrum for HD189733b between 2.0-2.4 mu m and 3.1-4.1 mu m, where we find a bright emission feature. Where overlap with space-based instruments exists, our results are in excellent agreement with previous measurements(2,6). A feature at similar to 3.25 mu m 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 fromnon-LTE emission from CH(4), similar to what is seen in the atmospheres of planets in our own Solar System(7-9). These results suggest that non-LTE effects may need to be considered when interpreting measurements of strongly irradiated exoplanets.