Effect of Core Cooling on the Radius of Sub-Neptune Planets

TL;DR

This study investigates how the cooling rate of the rocky core in sub-Neptune planets influences their overall radius evolution, challenging previous assumptions that core cooling has minimal impact.

Contribution

It introduces a model allowing variable core cooling times and demonstrates that core cooling can significantly affect planetary radii over Gyr timescales.

Findings

Core cooling can substantially increase planetary radius over billions of years.

The impact of core thermal properties introduces degeneracy in interpreting mass-radius data.

Better age estimates are needed to disentangle composition and thermal effects.

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 part (hereafter core) which is surrounded by a modest hydrogen-helium envelope. As a result, the total initial heat content of such a planet is dominated by that of the core. Nonetheless, most studies contend that the core cooling will only have a minor effect on the radius evolution of the gaseous envelope, because the core's cooling is in sync with the envelope, i.e., most of the initial heat is released early on timescales of about 10-100 Myr. In this Letter we examine the importance of the core cooling rate for the thermal evolution of the envelope. Thus, we relax the early core cooling assumption and present a model where the core is characterized by two parameters: the initial temperature and the cooling time. We find that core cooling can significantly enhance the radius of the planet when it operates on a timescale similar to the observed age, i.e. several Gyr. Consequently, the interpretation of sub-Neptunes' mass-radius observations depends on the assumed core thermal properties and the uncertainty therein. The degeneracy of composition and core thermal properties can be reduced by obtaining better estimates of the planet ages (in addition to their radii and masses) as envisioned by future observations.