Journal Article

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

Knutson, Heather A, Howard, Andrew W, Crossfield, Ian J. M, Beichman, Charles A, Wong, Ian, Isaacson, Howard, McCullough, P. R, Kubyshkina, Daria, Piaulet, Caroline, Peterson, Merrin S, Thorngren, Daniel, Morley, Caroline V, Dragomir, Diana, M.-R. Kempton, Eliza, Weiss, Lauren M, tney, Jonathan J, Almenara, Jose M, Benneke, Björn, Fossati, Luca, Angus, Ruth, Lammer, Helmut

2023

Overview

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 (Teq,AB=0.3≈350K) 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 (Md=2.1−0.7+0.6M⊕), as well as the flat optical/infrared transmission spectrum. Quantitatively, we infer a composition of 11−4+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 Rc = 1.51 ± 0.04 R⊕ and a Mc=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.A comprehensive study of the Kepler-138 system reveals the twin nature of Kepler-138 c and d and the presence of a fourth planet. Remarkably, the warm-temperate planet Kepler-138 d is probably composed of 50% volatiles by volume, indicative of a water world, rather than a rocky world, despite its small ~1.5 R⊕ size.

• Publisher: Nature Publishing Group
• Subject: Deep water / Earth / Planets / Silicates