Evidence for the volatile-rich composition of a 1.5-Earth-radius planet

The population of planets smaller than approximately 1.7 Earth radii (R_⊕) is widely interpreted as consisting of rocky worlds, generally referred to as super-Earths. This picture is largely corroborated by radial velocity mass measurements for close-in super-Earths but lacks constraints at lower insolations. 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 ± 0.04 R_⊕ planet Kepler-138 d (T_{e q ,AB=0.3}≈350 K ) combined with new radial velocity measurements of its host star obtained with the Keck/High Resolution Echelle Spectrometer. We find evidence for a volatile-rich `water world' nature of Kepler-138 d, with a large fraction of its mass $M_{{d}}$ contained in a thick volatile layer. This finding is independently supported by transit timing variations and radial velocity observations (M_d=2 .1_−0.7^+0.6M_⊕ ), as well as the flat optical/infrared transmission spectrum. Quantitatively, we infer a composition of 1 1₋₄⁺³% volatiles by mass or ~51% by volume, with a 2,000-km-deep water mantle and atmosphere on top of a core with an Earth-like silicates/iron ratio. Any hypothetical hydrogen layer consistent with the observations (<0.003 M_⊕) would have swiftly been lost on a ~10 Myr timescale. The bulk composition of Kepler-138 d therefore resembles those of the icy moons, rather than the terrestrial planets, in the Solar System. We conclude that not all super-Earths are rocky worlds, but that volatile-rich water worlds exist in an overlapping size regime, especially at lower insolations. Finally, our photodynamical analysis also reveals that Kepler-138 c (with a R_c = 1.51 ± 0.04 R_⊕ and a M_c=2 .3_−0.5^+0.6M_⊕ ) is a slightly warmer twin of Kepler-138 d (that is, another water world in the same system) and we infer the presence of Kepler-138 e, a likely non-transiting planet at the inner edge of the habitable zone.