Water content trends in K2-138 and other low-mass multiplanetary systems

Abstract

Context. Both rocky super-Earths and volatile-rich sub-Neptunes have been found simultaneously in multiplanetary systems, suggesting that these systems are appropriate to study different composition and formation pathways within the same environment. Aims. We perform a homogeneous interior structure analysis of five multiplanetary systems to explore the compositional trends and its relation with planet formation. For one of these systems, K2-138, we present revised masses and stellar host chemical abundances to improve the constraints on the interior composition of its planets. Methods. We conduct a line-by-line differential spectroscopic analysis on the stellar spectra of K2-138 to obtain its chemical abundances and the planetary parameters. We select multiplanetary systems with five or more low-mass planets (M < 20 M ⊕ ) that have both mass and radius data available. We carry out a homogeneous interior structure analysis on the planetary systems K2-138, TOI-178, Kepler-11, Kepler-102 and Kepler-80. We estimate the volatile mass fraction of the planets in these systems assuming a volatile layer constituted of water in steam and supercritical phases. Our interior-atmosphere model takes into account the effects of irradiation on the surface conditions. Results. K2-138 inner planets present an increasing volatile mass fraction with distance from its host star, while the outer planets present an approximately constant water content. This is similar to the trend observed in TRAPPIST-1 in a previous analysis with the same interior-atmosphere model. The Kepler-102 system could potentially present this trend. In all multiplanetary systems, the low volatile mass fraction of the inner planets could be due to atmospheric escape while the higher volatile mass fraction of the outer planets can be the result of accretion of ice-rich material in the vicinity of the ice line with later inward migration. Kepler-102 and Kepler-80 present inner planets with high core mass fractions which could be due to mantle evaporation, impacts or formation in the vicinity of rocklines.