Interior Structure and Chemistry of Solid Exoplanets

Abstract

Understanding the interior structures and chemistry of Earth-like exoplanets is crucial for us to characterize exoplanets, and to find potentially habitable planets.

First, I provide a model grid of the mass-radius relations for solid planets in between 0.1 and 100 Earth masses. Planets are modeled as consisting of three layers: Fe, MgSiO3 and H2O. This model is made into an interactive tool available online: http://www.astrozeng.com/

Second, I explore the effects of thermal evolution and phase transitions on the interior structures of H2O-rich planets. It is shown that the bulk H2O in such planets may exist in the plasma, superionic, ionic, Ice VII, or Ice X states depending on sizes, ages, and cooling rates. The results suggest that super-Earth sized planets which are not significantly irradiated by parent stars and which are older than approximately 3 billion years, are mostly solid.

Third, I describe a new, semi-empirical mass-radius relation for solid exoplanets. It is based on the recent mass and radius measurements of 5 exoplanets within 1 to 10 Earth masses and an extrapolation of the seismically derived pressure-density relation of the Earth's interior (PREM). The implication of common core mass fractions of 0.2~0.3 among these solid exoplanets is also discussed.

Fourth, I model the elemental abundance patterns of solid exoplanets based on that of their host stars. This model is constructed from the following steps of planet formation: volatile depletion, core formation, and late delivery. This model could provide constraints on the chemical compositions of solid exoplanets in addition to the constraints derived from their masses and radii.

In terms of future directions of this research, I hope to link my chemical model of solid exoplanets with the chemical evolution model of our galaxy, such as the one being developed by the Lars Hernquist group, which may indicate a different mineralogy of solid exoplanets formed at different ages of our galaxy, as well as the implications for the habitability of these planets. I also hope to understand the origins of the volatile contents on the surfaces of solid planets, which are important prerequisites for possible origins of life on them.