Water Vapour in the Atmospheres of Hot-Jupiters
Hot-Jupiters are a class of planets similar in size to Jupiter but orbit very close to their host stars. It is common for hot-Jupiters to have cloud or haze layers in their atmospheres that can mask the detection of atmospheric constituents that lie deeper down in the atmosphere. These cloud or haze layers have been found to be concealing the presence of water vapour in the atmospheres of hot-Jupiters.
When a transiting planet passes in front of its host star, it blocks some of the light from its host star. The size of the transiting planet can be determined by measuring the amount of starlight it blocks. If the transiting planet is cloudy or hazy, it is expected to appear larger when observed in optical wavelengths than in infrared wavelengths. This is because infrared radiation can penetrate clouds more readily than optical radiation (i.e. visible light). As a result, the difference in the planet's radius when its transit is observed in optical and infrared wavelengths can be used to determine whether the planet's atmosphere is cloudy or clear.
Using data from the Hubble Space Telescope (HST), Iyer et al. (2016) present a study of 19 hot-Jupiters. 10 of these hot-Jupiters show the presence of water vapour in their atmospheres, while the other 9 hot-Jupiters show no water vapour at all. The datasets for all 19 hot-Jupiters were combined to create one average overall spectrum. The overall spectrum agrees with models showing that cloud and haze layers are hiding the presence of water vapour deeper in the atmosphere. Such a finding means that hot-Jupiters did not form in an environment deprived of water.
Iyer et al. (2016), "A Characteristic Transmission Spectrum dominated by H2O applies to the majority of HST/WFC3 exoplanet observations", arXiv:1512.00151 [astro-ph.EP]