1. European Space Agency, Frascati 00044, Italy
2. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
3. Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, 75014 Paris, France
4. Division of Geological and Planetary Sciences,
5. IPAC-Spitzer Science Center, California Institute of Technology, Pasadena, California 91125, USA
6. Institut de Ciències de l'Espai (CSIC-IEEC), Campus UAB, 08193 Bellaterra, Spain
7. Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, 85721-0063, Arizona, USA
8. Centre National d'Etudes Spatiales, Paris 75001, France
9. Ecole Normale Supérieure, CNRS, 69364 Lyon, France

Correspondence to: Giovanna Tinetti^1,2,3 Correspondence and requests for materials should be addressed to G.T. (Email: gio@gps.caltech.edu).

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 it^{3, 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 m, 5.8 m (both ref. 7) and 8 m (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.

1. European Space Agency, Frascati 00044, Italy
2. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
3. Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, 75014 Paris, France
4. Division of Geological and Planetary Sciences,
5. IPAC-Spitzer Science Center, California Institute of Technology, Pasadena, California 91125, USA
6. Institut de Ciències de l'Espai (CSIC-IEEC), Campus UAB, 08193 Bellaterra, Spain
7. Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, 85721-0063, Arizona, USA
8. Centre National d'Etudes Spatiales, Paris 75001, France
9. Ecole Normale Supérieure, CNRS, 69364 Lyon, France

Correspondence to: Giovanna Tinetti^1,2,3 Correspondence and requests for materials should be addressed to G.T. (Email: gio@gps.caltech.edu).

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