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

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The relatively young research field of extrasolar planets has revealed an unexpectedly large diversity of planets, which could not be foreseen from studies of the Solar System. After the astonishing discovery of Hot Jupiters, which can not be formed where they are observed, the finding that no clear division into rocky and gaseous planets is realized, has revolutionized our understanding of planetary formation and evolution. The rather continuous transition between rocky and gaseous planets is found in the sub-Neptunian mass and radius regime (M<20M_Earth and R<4R_Earth), and was identified by independent measurements of planetary radius and mass. The resulting mean density of planets in these regime can be accounted for in terms of a variety of interior structures based on an iron core, a rocky mantle, possibly an ice layer, and a more or less thick atmosphere, all of them in proportions that are difficult to constrain. The atmospheres of these planets may have different origins, such as accretion from the protoplanetary disc, sublimation of accreted ices, or outgassing from the interior. In this project, we aim at understanding for which detected planets in the sub-Neptunian mass-radius regime secondary outgassing from rocky interiors due to mantle melting and volcanism is in principle possible. Furthermore, we are interested in understanding how large the impact of interior-atmosphere feedbacks due to the outgassing and subsequent changes in the atmospheric conditions and escape may be on the measured planetary radius. We will expand models of interior structure, thermal and outgassing evolution, as well as atmosphere models and apply them to observed planets in the sub-Neptunian mass-radius regime. The inferred interior structures will build the starting point of investigations of the interior thermal evolution and potential outgassing, as well as of the atmospheric temperature and composition. We expect that outgassing from the interior, of CO2 in particular, may lead to important changes in the atmospheric mass, composition and, in turn, also atmospheric temperatures. Together with collaboration partners, we will explore whether these modeled atmospheres are subject to strong atmospheric escape, and under which circumstances CO2-outgassing may alter the loss processes. The feedback of the atmospheric temperature and mass upon the interior evolution will be finally considered by building a coupled interior-atmosphere model.The results of these detailed model calculations of the planetary interior and atmosphere will be compared to the initial results of the interior structure modelling to show under which conditions these processes may substantially modify the apparent planetary radius. This may allow us to rule out certain solutions derived from interior structure models alone and to put additional constraints on the nature of the investigated bodies.