Core-powered mass loss and the radius distribution of small exoplanets

TL;DR

This paper proposes that the cooling of rocky exoplanet cores can explain the observed bimodal radius distribution of small exoplanets, offering an alternative to stellar high-energy radiation-driven atmospheric erosion.

Contribution

It introduces a model of core-powered mass loss that self-consistently accounts for envelope evolution, successfully reproducing the observed radius distribution without relying on stellar high-energy flux.

Findings

Core-powered mass loss can produce the observed radius valley.

The model matches observed distributions across different stellar types.

It provides an alternative explanation to photoevaporation for atmospheric loss.

Abstract

Recent observations identify a valley in the radius distribution of small exoplanets, with planets in the range $1.5-2.0\,{\rm R}_{\oplus}$ significantly less common than somewhat smaller or larger planets. This valley may suggest a bimodal population of rocky planets that are either engulfed by massive gas envelopes that significantly enlarge their radius, or do not have detectable atmospheres at all. One explanation of such a bimodal distribution is atmospheric erosion by high-energy stellar photons. We investigate an alternative mechanism: the luminosity of the cooling rocky core, which can completely erode light envelopes while preserving heavy ones, produces a deficit of intermediate sized planets. We evolve planetary populations that are derived from observations using a simple analytical prescription, accounting self-consistently for envelope accretion, cooling and mass loss, and demonstrate that core-powered mass loss naturally reproduces the observed radius distribution, regardless of the high-energy incident flux. Observations of planets around different stellar types may distinguish between photoevaporation, which is powered by the high-energy tail of the stellar radiation, and core-powered mass loss, which depends on the bolometric flux through the planet's equilibrium temperature that sets both its cooling and mass-loss rates.