Thermal evolution of sub-Neptunes: the role of the rocky core
Abstract
Sub-Neptune planets are very common in our Galaxy and show a large diversity in their mass-radius relation. In sub-Neptunes most of the planet mass is in the rocky core, which is surrounded by a modest hydrogen-helium envelope. We study the long-term consequences of the core cooling on the planet mass-radius relation. We consider the role of various core energy sources resulting from core formation, iron differentiation, rock solidification, core contraction, and radioactive decay. We follow the core formation phase, which sets the initial conditions, the magma ocean phase, characterised by rapid heat transport, and the solid-state phase, where cooling is inefficient. We find that for typical sub-Neptune planets (2 - 10 Earth masses) with envelope mass of 0:5% - 10%, the magma ocean phase lasts several gigayears, much longer than for terrestrial planets. The magma ocean phase effectively erases any signs of the initial core thermodynamic state. After solidification, the reduced heat flux from the rocky core causes a significant drop in the rocky core surface temperature, but its effect on the planet radius is limited. The overall long-term radius uncertainty by core effects is usually about 5%, and not more than 15%. Therefore, the inferred envelope mass from mass-radius relation is mostly proportional to the envelope (H/He) mass fraction.
- Publication:
-
EPSC-DPS Joint Meeting 2019
- Pub Date:
- September 2019
- Bibcode:
- 2019EPSC...13..219V