Characterising the interior structures and atmospheres of transiting super-Earths and sub-Neptunes

The modelling of the internal structures of super-Earths and sub-Neptunes gives a valuable insight into their formation history and possible atmospheres. We present a planet model where the interior is coupled with the atmosphere within a Bayesian retrieval scheme. We take into account water in all its possible phases, including steam and supercritical phases, which is necessary for systems with a wide range of stellar irradiations. Our interior-atmosphere model calculates the compositional and atmospheric parameters, such as Fe and water content, surface pressures, scale heights and albedos. We analyse the highly-irradiated planet TOI-220 b, and the multiplanetary systems K2-138 and TRAPPIST-1. The very low density of TOI-220 b can only be explained with an extended atmosphere, which could be dominated by water or H/He as well. For TRAPPIST-1 and K2-138,  we derive with their individual compositions a global increasing trend on the water content with increasing distance from the star in the inner region of the systems, while the planets in the outer region present a constant water mass fraction. This trend reveals the possible effects of migration, formation location and atmospheric mass loss during their formation history.