Water vapour in the atmosphere of a transiting extrasolar planet

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Abstract

Water is predicted to be among the most abundant (if not the most abundant) molecular species after hydrogen in the atmospheres of close-in extrasolar giant planets (‘hot Jupiters’)1,2. Several attempts have been made to detect water on such planets, but have either failed to find compelling evidence for it3,4 or led to claims that should be taken with caution5. Here we report an analysis of recent observations of the hot Jupiter HD 189733b (ref. 6) taken during the transit, when the planet passed in front of its parent star. We find that absorption by water vapour is the most likely cause of the wavelength-dependent variations in the effective radius of the planet at the infrared wavelengths 3.6 μm, 5.8 μm (both ref. 7) and 8 μm (ref. 8). The larger effective radius observed at visible wavelengths9 may arise from either stellar variability or the presence of clouds/hazes. We explain the report of a non-detection of water on HD 189733b (ref. 4) as being a consequence of the nearly isothermal vertical profile of the planet’s atmosphere.

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Figure 1: A comparison of the observations with simulated water absorption.
Figure 2: A comparison of the observations of primary transit with a simulated infrared and optical transmission spectrum.
Figure 3: Simulated emission spectra of HD 189733b in the infrared.

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Acknowledgements

We thank A. Lecavelier, D. Ehrenreich, J.-M. Désert, Roger Ferlet and G. Hebrard for their work on the IRAC observations. We thank A. Noriega-Crespo and the Spitzer Staff for helping to schedule the observations with IRAC, and E. Lellouch, A. Morbidelli, B. Schultz, F. Bouchy and J. B. Marquette for useful input to the paper. M.-C.L. and Y.Y. were supported by NASA and the Virtual Planetary Laboratory at the California Institute of Technology.

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Correspondence to Giovanna Tinetti.

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