Influence of partially molten layers on the tidal response of rocky exoplanets

The number of detected Earth-sized exoplanets is now increasing, and most of the detected planets orbit at relatively close distance from their host stars, resulting in strong tidal forcing. As shown for the inner planets of the Trappist-1 system [1] or the newly discovered Earth-sized exoplanet LP 791-18d [2], tidal heating is expected to be a dominant source of heating, potentially exceeding the radiogenic power by one order of magnitude and more. Depending on the orbital eccentricity, tidally-induced thermal runaways may result in strong internal melting and volcanic heat flux comparable to Io [3]. As shown in the case of Io, the presence of silicate melt in the interior of Io has a strong influence on the tidal response of its interior [3,4]. The thickness and melt content of partially molten layer can strongly affect the total dissipated power and its distribution.

In this study, we test the influence of partially molten layers on the tidal response of Earth-sized exoplanets, using Trappist-1 b,c and d and LP 791-18d as examples.  We follow the approach developed to model the solid tides in Io’s partially molten interior [3], taking into account the effect of melt on the viscoelastic properties of the mantle, and test different rheological models (Maxwell, Andrade, Sundberg-Cooper). We use interior structure models consistent with the estimated mass and radius and consider partially molten layers of various thickness,  depth and melt content. Results for various assumptions in interior composition and melt content will be presented and implications for the heat budget of these planets  will be discussed.

[1] Turbet et al. A&A 612, A86;  [2] Peterson et al.  Nature, 617, 701–705 (2023);  [3] Běhounková et al. ApJ, 728(2), 89 (2011) ; [4] Kervazo et al., A&A, 650, A72 (2021) ;  [5] Kervazo et al. Icarus, 373, 114737 (2022).