Interior-atmosphere feedbacks and the nature of detected sub-Neptunian planets

One of the major quests in exoplanetary science today is understanding the diversity of so-called sub-Neptunes. With masses and radii below about 20 Earth masses and 4 Earth radii, these relatively small planets show a large variety of mean densities that are compatible with qualitatively different interior and atmosphere structures: rocky planets with extended hydrogen envelopes, ocean planets with water-dominated atmospheres, or mini gas planets with large solid cores can match equally well observations of planetary mass and radius. Characterizing these planets on the basis of these observations only is not an easy task due to the large number of equally admissible structures of the interior and atmosphere.  However, some of the structures consistent with the observations are less likely than others, for example those with atmospheres that would be unstable against escape processes, or pure water or iron spheres.
For certain interior structures compatible with the observations, an interaction between the interior and atmosphere via outgassing – the release of volatiles from the mantle via volcanism – can be expected.  Upon studying this interaction several factors need to be considered. The evolution of the mantle and core is influenced by the different interior and atmosphere structures compatible with the measured mass and radius, by the composition, volatile content and mechanical  properties of the mantle, as well as by its thermal state. Depending on the surface temperature, the rocky mantle at the interface with the atmosphere can either be solid or molten, which leads to qualitative different behaviours in the thermal and outgassing evolution of a planet. On the one hand,  outgassing can change the composition and mass of the atmosphere, which controls the surface temperature. On the other hand, outgassing can also be strongly influenced by the atmosphere itself, whose pressure exerted at the surface affects the solubility of volatiles in magmas and in turn the efficiency with which these can be released.
In this project, we have been designing a new code named TATOOINE for Tool for ATmospheres, Outgassing and Optimal INteriors of Exoplanets, which is complemented by an efficient machine-learning algorithm that allows us to rapidly infer the possible distribution of interior and atmosphere structures compatible with given observed mass and radius. Given the admissible solutions, we then investigate those for which outgassing from the interior is possible and study the co-evolution of the interior and atmosphere including atmospheric escape, as well as the evolution of the host star. Through this approach, we will evaluate whether or not interior-atmosphere feedbacks can change the planetary radius so severely that the interior structures inferred without accounting for these feedbacks can be disregarded.