Thermal evolution of rocky exoplanets with a graphite outer shell

TL;DR

This study models how a graphite outer shell affects the thermal evolution of rocky exoplanets, showing that graphite's high thermal conductivity leads to faster cooling compared to silicate lids, with implications for planetary thermal history.

Contribution

It introduces a parameterized model to evaluate the impact of a graphite outer shell on exoplanet thermal evolution, highlighting the conductive nature of graphite as a stagnant lid.

Findings

Graphite shells conduct heat efficiently, dominating long-term heat transport.

Planets with graphite lids cool faster than those with silicate lids.

Thicker graphite lids are needed to match silicate lid thermal effects.

Abstract

The presence of rocky exoplanets with a large refractory carbon inventory is predicted by chemical evolution models of protoplanetary disks of stars with photospheric C/O >0.65, and by models studying the radial transport of refractory carbon. High-pressure high-temperature laboratory experiments show that most of the carbon in these exoplanets differentiates into a graphite outer shell. Our aim is to evaluate the effects of a graphite outer shell on the thermal evolution of rocky exoplanets containing a metallic core and a silicate mantle. We implement a parameterized model of mantle convection to determine the thermal evolution of rocky exoplanets with graphite layer thicknesses up to 1000 km. We find that, due to the high thermal conductivity of graphite, conduction is the dominant heat transport mechanism in a graphite layer for the long-term evolution (>200 Myr). The conductive graphite shell essentially behaves like a stagnant lid with a fixed thickness. Models of Kepler-37b (Mercury-size) and a Mars-sized exoplanet show that a planet with a graphite lid cools faster than a planet with a silicate lid, and a planet without a stagnant lid cools the fastest. A graphite lid needs to be approximately ten times thicker than a corresponding silicate lid in order to produce similar thermal evolution.