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 , A B = 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 $M_{\rm{d}}$ 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 + 0.6 M ⊕ ), 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 (