Water vapour on a Neptune-sized exoplanet

Jonathan Fraine et al. have obtained the transmission spectrum of the Neptune-sized exoplanet HAT-P-11b from the optical to the infrared. In it they detected water vapour absorption at 1.4-micrometre wavelength. HAT-P-11b is the smallest and coldest planet with an absorption signature measured by transmission. Previously it had been possible to measure atmosphere compositions only for large, Jupiter-sized exoplanets, but this latest advance now means that it is possible to measure atmospheric mean molecular weight, providing insights into the formation history of planets as small as four Earth radii.