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

The population of planets smaller than approximately 1.7 Earth radii 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. I will 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 ± 0.04 Rearth planet Kepler-138d combined with 28 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 (M_d=2.1_-0.7^+0.6 M_earth), as well as the flat optical/IR transmission spectrum. Any hypothetical hydrogen layer consistent with the observations (lighter than 0.003 Mearth), would have swiftly been lost on ~10 Myr timescales. Quantitatively, we infer a composition of 11_-4⁺³% volatiles by mass or ~51% by volume, with a 2000 km deep water mantle and atmosphere on top of a core with an Earth-like silicates/iron ratio. This bulk composition resembles those of the icy moons rather than the terrestrial planets in the solar system. Finally, our photodynamical analysis also reveals that Kepler-138c is a closer-in twin of Kepler-138d, and we infer the presence of Kepler-138e, a likely non-transiting planet at the inner edge of the habitable zone.