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.
Highlights
Multiplanetary systems appear to be suitable distant laboratories to explore the diversity of small planets, and their formation and evolution pathways
K2-138 presents a very clear volatile mass fraction trend: an increasing gradient in water content with distance from the host star for planets b to d, followed by a constant volatile mass fraction for the outer planets
Assuming that the planetary Fe/Si mole ratio is similar to the Fe/Si ratio of the host star improves the determination of the core mass fraction (CMF), but does not necessarily contribute to the determination of the volatile mass fraction in volatile-rich planets (Otegi et al 2020)
Summary
Multiplanetary systems appear to be suitable distant laboratories to explore the diversity of small planets, and their formation and evolution pathways This is the case of Kepler-36 (Carter et al 2012), where its two planets, b and c, present periods of 14 and 16 days with densities of 7.5 and 0.9 g cm−3, respectively. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 198.C-0.168 Their systems that include the precision reached on the fundamental parameters of both the planets and the star, and the different assumptions considered between different interior structure models. In our interior structure model, we consider that the volatile layer is water-dominated, following the approach of Mousis et al (2020) and Acuña et al (2021) This analysis allows us to uncover volatile and core mass fraction trends, and their connection with planet formation and evolution.
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