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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 (H2O), methane (CH4), carbon dioxide (CO2), and carbon monoxide (CO) have been detected1,2,3,4,5 in two hot Jupiters. These previous results relied on space-based telescopes that do not provide spectroscopic capability in the 2.4–5.2 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 measurements2,6. 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 System7,8,9. These results suggest that non-LTE effects may need to be considered when interpreting measurements of strongly irradiated exoplanets.

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Figure 1: Dayside spectra and secondary eclipse light curves.
Figure 2: Unexpectedly strong 3.25-μm emission present in the dayside spectrum.


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We thank S. Bus at the IRTF for several discussions regarding the operation of the SpeX instrument and for support during our observing runs. We thank the observing staff at the IRTF for their assistance and advice during observing runs. We thank L. Brown for making recommendations on molecular line lists and G. Orton for extensive discussions about the interpretation of these results. G. Tinetti was supported by the UK Sciences and Technology Facilities Council and the European Space Agency. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

Author Contributions M.R.S. wrote the paper, participated in developing the calibration methods, and wrote the telescope proposals. P.D. worked on developing and testing the calibration method. C.A.G. and G.T. worked on the interpretation of the results. A.T. worked on the calibration method. G.V. and P.C. worked on calibration validation. J.B. worked on the calibration of the Spitzer data. I.J.C. worked on an early version of the calibration method. D.A. contributed to the atmospheric calibration approach. C.A. and T.H. were co-authors on the original telescope proposal and provided comments on the paper text.

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Correspondence to Mark R. Swain.

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Swain, M., Deroo, P., Griffith, C. et al. A ground-based near-infrared emission spectrum of the exoplanet HD 189733b. Nature 463, 637–639 (2010).

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