Not all super-Earths are rocky planets: evidence for a warm-temperate volatile-rich water world

Caroline Piaulet ( Université de Montréal )

The population of planets smaller than approximately 1.7 Rearth is widely interpreted as consisting of rocky worlds. This picture is largely corroborated by radial-velocity (RV) mass measurements for close-in super-Earths, but lacks constraints at lower instellations. Here we present the results of a detailed study of the Kepler-138 system using 13 Hubble and Spitzer transit observations of the warm-temperate 1.51 Rearth planet Kepler-138d combined with Keck/HIRES RV measurements of its host star. We find evidence for a volatile-rich “water world” nature of Kepler-138d, independently supported by transit timing variations, RV observations, as well as the flat optical/IR transmission spectrum. The bulk composition of Kepler-138d resembles those of the icy moons rather than the terrestrial planets in the solar system. Our photodynamical analysis provides important revisions of the parameters of the three known small planets in the system and we infer the presence of Kepler-138e, a likely non-transiting planet at the inner edge of the habitable zone.