Molecules present in the atmospheres of extrasolar planets are expected to influence strongly the balance of atmospheric radiation, to trace dynamical and chemical processes, and to indicate the presence of disequilibrium effects. As molecules have the potential to reveal atmospheric conditions and chemistry, searching for them is a high priority. The rotational–vibrational transition bands of water, carbon monoxide and methane are anticipated to be the primary sources of non-continuum opacity in hot-Jupiter planets1,2,3. As these bands can overlap in wavelength, and the corresponding signatures from them are weak, decisive identification requires precision infrared spectroscopy. Here we report a near-infrared transmission spectrum of the planet HD 189733b that shows the presence of methane. Additionally, a resolved water vapour band at 1.9 μm confirms the recent claim4 of water in this object. On thermochemical grounds, carbon monoxide is expected to be abundant in the upper atmosphere of hot-Jupiter planets, but is not identifiable here; therefore the detection of methane rather than carbon monoxide in such a hot planet5,6 could signal the presence of a horizontal chemical gradient away from the permanent dayside, or it may imply an ill-understood photochemical mechanism that leads to an enhancement of methane.
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We thank D. Deming for contributions to the original proposal and for suggestions for clarifying the material presented in the manuscript. We thank T. Wiklind, N. Pirzkal and other members of the Space Telescope Science Institute staff for assistance in planning the observations and for advising how the observations could be optimized. We also thank K. Jahnke for suggesting the long-wavelength NICMOS grism, F. Pont for discussions concerning the treatment of the data, M.-C. Liang for discussions on photolysis, and J. Tennyson and R. Barber for suggestions on the methane data lists. G.T. was supported by the UK Sciences & 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 NASA.
Author Contributions M.R.S. was the PI of the project and led the overall direction of the research. G.V. led the data analysis and G.T. led the modelling. All authors contributed equally to the writing of the paper.
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Swain, M., Vasisht, G. & Tinetti, G. The presence of methane in the atmosphere of an extrasolar planet. Nature 452, 329–331 (2008). https://doi.org/10.1038/nature06823
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