Contribution of the Core to the Thermal Evolution of Sub-Neptunes

Sub-Neptune planets are a very common type of planet. They are inferred to harbor a primordial (H/He) envelope on top of a (rocky) core, which dominates the mass. Here, we investigate the long-term consequences of the core properties on the planet mass–radius relation. We consider the role of various core energy sources resulting from core formation, its differentiation, its solidification (latent heat), core contraction, and radioactive decay. We divide the evolution of the rocky core into three phases: the formation phase, which sets the initial conditions, the magma ocean phase, characterized by rapid heat transport, and the solid-state phase, where cooling is inefficient. We find that for typical sub-Neptune planets of ~2–10 M ⊕ and envelope mass fractions 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. In the long run, radioactive heating is the most significant core energy source in our model. Overall, the long-term radius uncertainty by core thermal effects is up to 15%.