The three regimes of atmospheric evaporation for super-Earths and sub-Neptunes

TL;DR

This paper presents a unified framework describing three distinct regimes of atmospheric evaporation in super-Earths and sub-Neptunes, based on internal temperature and irradiation effects, clarifying the mechanisms and timescales of mass loss.

Contribution

It introduces the first comprehensive model unifying different atmospheric evaporation regimes over a planet's lifespan, considering internal temperature and irradiation effects.

Findings

Three regimes of atmospheric loss identified based on internal temperature and irradiation.

High-temperature regime involves efficient convective mass loss without cooling.

Low-temperature regime is dominated by X-ray and ultraviolet irradiation effects.

Abstract

A significant fraction of super-Earths and sub-Neptunes are thought to experience an extreme loss of volatiles because of atmospheric evaporation in the early stages of their life. Though the mechanisms behind the extreme mass loss are not fully understood, two contenders have been widely discussed: photoevaporation from X-ray and ultraviolet irradiation and core powered mass loss. Here, it is shown that both mechanisms occur but with different timescales, and that atmospheric loss can take place over three regimes. In the first regime, a planet has very high internal temperatures arising from its high-energy formation processes. These high temperatures give rise to a fully convecting atmosphere that efficiently loses mass without much internal cooling. The second regime applies to planets with lower internal temperatures, so a radiative region forms but the photosphere still remains outside the Bondi radius. Hence, mass loss continues to depend only on the internal temperatures. Planets with the lowest internal temperatures are in the third regime, when the photosphere forms below the Bondi radius and mass is lost primarily because of X-ray and ultraviolet irradiation. This paper provides the first unifying framework for modeling atmospheric evaporation through the lifespan of a planet.