Initial Thermal States of Super-Earth Exoplanets and Implications for Early Dynamos

Abstract

The accretion of Earth and the formation of a metallic core released a large amount of primordial heat and may have enabled its evolution into a habitable world. Metal-silicate segregation likely occurs in super-Earth exoplanets as well, but its influence on their initial thermal states has not been fully examined. Here we calculated the energy released during core-mantle differentiation of super-Earths for a range of planet radii and core mass fractions. We found that the energy of differentiation increases with planet mass for rocky planets with Earth-like composition, and it peaks at 55% core by mass in Earth-sized rocky planets. Using the latest mineral physics constraints on the equations-of-state and melting curve of relevant phases, we modeled the initial thermal profiles and assessed the extent of melting in initial iron cores for plausible heat retention efficiencies. Our results suggest that following accretion and metal-silicate differentiation, the cores of most super-Earths are expected to be at least partially molten, a necessary condition for the generation of a magnetic field. Based on the largely molten state of Earth's core at the present day, we place a lower bound of 7% retention of accretional energy as primordial heat in rocky planets.