Atmospheres of sub-Neptune-sized exoplanets in contact with magma ocean
speaker: Seo Chanoul
Abstract:
Sub-Neptunes are small exoplanets (Planetary radius = 2~3 Earth radius) with voluminous atmospheres and are important targets of future recent and upcoming transit spectroscopy. However, the framework to relate their atmospheric properties to their hidden interior and the formation pathway has not been well established. As the mass of sub-Neptunes is dominated by the silicate core, their atmospheres are likely to be affected by the reaction with the underlying magma (i.e., molten rock) if they exist. Thus, we study the atmospheric composition of sub-Neptunes in the presence of magma ocean using an equilibrium model of the Fe-Si-Mg-H-C-O system with Fe-FeO-Fe2O3 magma redox buffer. We find the nebula-origin atmospheres can be highly oxidized to form H2O-dominated atmospheres when the accreted amount of volatile is moderate and(or) the iron core fraction is small. We show how the two observable atmospheric parameters, H2O fraction and atmospheric C/H ratio, in the combination of the measurement of mass and radius, can infer the initial magma redox state and composition of accreted volatiles (nebula gas and pure ice). The high solubility of H2O into magma and the small solubility of C-bearing species enrich C-bearing species in the atmosphere, which could be used as an indicator of the presence of magma-atmospheric reaction. Such properties of atmospheres may also provide a way to distinguish several suggested formation scenarios of small planets around M-type stars.