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

摘要

Detection of molecules using infrared spectroscopy probes the conditions andcompositions of exoplanet atmospheres. Water (H2O), methane (CH4), carbondioxide (CO2), and carbon monoxide (CO) have been detected in two hot Jupiters.These previous results relied on space-based telescopes that do not providespectroscopic capability in the 2.4 - 5.2 micron spectral region. Here wereport ground-based observations of the dayside emission spectrum for HD189733b between 2.0-2.4 micron and 3.1-4.1 micron, where we find a brightemission feature. Where overlap with space-based instruments exists, ourresults are in excellent agreement with previous measurements. A feature at~3.25 micron is unexpected and difficult to explain with models that assumelocal thermodynamic equilibrium (LTE) conditions at the 1 bar to 1 x 10-6 barpressures typically sampled by infrared measurements. The most likelyexplanation 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 wheninterpreting measurements of strongly irradiated exoplanets.