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

Abstract

Context. Both rocky super-Earths and volatile-rich sub-Neptunes have been found simultaneously in multi-planetary 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 multi-planetary systems to explore compositional trends and their 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 conducted a line-by-line differential spectroscopic analysis on the stellar spectra of K2-138 to obtain its chemical abundances and the planetary parameters. We selected multi-planetary systems with five or more low-mass planets (M < 20 M⊕) that have both mass and radius data available. We carried out a homogeneous interior structure analysis on the planetary systems K2-138, TOI-178, Kepler-11, Kepler-102, and Kepler-80. We estimated 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 took the effects of irradiation on the surface conditions into account. Results. K2-138 inner planets present an increasing volatile mass fraction with distance from their 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 multi-planetary 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.