Water vapor and liquid water clouds on the habitable-zone exoplanet K2-18b

Results from the Kepler mission indicate that the occurrence rate of habitable-zone Earths and super-Earths may be as high as 5-20%. Despite this abundance, probing the conditions and atmospheric properties on any of these habitable-zone planets is extremely difficult and has remained elusive to date. Here, we report the detection of water vapor and the likely presence of liquid water clouds in the atmosphere of the 8.6 Earth-mass habitable-zone planet K2-18b. With a 33 day orbit around a cool M3 dwarf, K2-18b receives virtually the same amount of total radiation from its host star (1441±80 W/m²) as the Earth receives from the Sun (1370 W/m²), making it a good candidate to host liquid water clouds. In this study, our team observed eight transits using HST/WFC3 in order to achieve the necessary sensitivity to detect water vapor and clouds. While the thick gaseous envelope of K2-18b means that it is not a true Earth analog, our observations demonstrate that low-mass habitable-zone planets with the right conditions for liquid water are accessible with state-of-the-art telescopes.