  Water is predicted to be among, 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 an exoplanet,
but have 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 HD189733b taken during the transit, where 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, 5.8 and 8 microns. The larger
effective radius observed at visible wavelengths may be due to either star
variability or the presence of clouds/hazes. We explain the most recent thermal
infrared observations of the planet during secondary transit behind the star,
reporting a non-detection of water on HD189733b, as being a consequence of the
nearly isothermal vertical profile of the planet.s atmosphere. Our results show
that water is detectable on extrasolar planets using the primary transit
technique and that the infrared should be a better wavelength region than the
visible, for such searches.
