title: Water vapour in the atmosphere of a transiting extrasolar planet.
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'). Several attempts have been made to detect water on such planets, but have either failed to find compelling evidence for it or led to claims that should be taken with caution. 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 mum, 5.8 mum (both ref. 7) and 8 mum (ref. 8). The larger effective radius observed at visible wavelengths 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.
authors.affiliation: European Space Agency, Frascati 00044, Italy. gio@gps.caltech.edu
authors.forename: Giovanna
authors.initials: G
authors.lastname: Tinetti
authors.forename: Alfred
authors.initials: A
authors.lastname: Vidal-Madjar
authors.forename: Mao-Chang
authors.initials: MC
authors.lastname: Liang
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Beaulieu
authors.forename: Yuk
authors.initials: Y
authors.lastname: Yung
authors.forename: Sean
authors.initials: S
authors.lastname: Carey
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Barber
authors.forename: Jonathan
authors.initials: J
authors.lastname: Tennyson
authors.forename: Ignasi
authors.initials: I
authors.lastname: Ribas
authors.forename: Nicole
authors.initials: N
authors.lastname: Allard
authors.forename: Gilda E
authors.initials: GE
authors.lastname: Ballester
authors.forename: David K
authors.initials: DK
authors.lastname: Sing
authors.forename: Franck
authors.initials: F
authors.lastname: Selsis
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D001272:Atmosphere / Q000737:chemistry*; D018559:Exobiology; D005740:Gases / Q000032:analysis*; D016083:Planets; D013057:Spectrum Analysis; D014867:Water / Q000032:analysis*
chemical_list: D005740:Gases; D014867:Water