DEPARTMENT OF PHYSICS AND ASTRONOMY

M. R. Swain, P. Deroo, C. A. Griffith, G. Tinetti, A. Thatte, G. Vasisht, P. Chen, J. Bouwman, I. J. Crossfield, D. Angerhausen, C. Afonso & T. Henning
Nature 463, 637-639

Astronomers have discovered a new ground-based technique to study the atmospheres of planets outside our Solar System, accelerating our search for Earth-like planets with life-related molecules.

Detection of molecules via infrared spectroscopy probes the conditions and composition of exoplanet atmospheres. To date, water (H2O), methane (CH4), carbon dioxide (CO2), and carbon monoxide (CO) have been detected in three hot-Jupiter type exoplanets.

These previous results relied on space-based telescopes that do not provide spectroscopic capability in the important 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. The feature around 3.25 μm is completely unexpected and is difficult to explain with models that assume local thermodynamic equilibrium conditions in the 1 bar to 1×10-6 pressures typically sampled by infrared measurements. The most likely explanation for this emission is non-LTE emission from CH4, similar to what is seen in the atmosphere of planets in our own solar system.