Habitable Zone and Atmosphere Retention Distance (HaZARD) Stellar-evolution-dependent loss models of secondary atmospheres

TL;DR

This study models the atmospheric retention distances for Earth-sized exoplanets around various stars, revealing how stellar evolution and initial rotation influence the likelihood of secondary atmosphere retention, impacting exoplanet habitability assessments.

Contribution

It introduces a combined stellar evolution and atmospheric escape model to predict atmospheric retention zones around different stellar types, considering stellar rotation and activity.

Findings

Atmospheric retention zones overlap with habitable zones earlier around slowly rotating stars.

Stars under 0.4 solar masses are unlikely to retain atmospheres on orbiting planets.

JWST-observed Earth-like exoplanets are outside the atmospheric retention distance.

Abstract

A major open question in exoplanet research is whether secondary atmospheres are rare around Earth-sized rocky exoplanets. In this work we determine the distance at which an Earth-sized planet orbiting a variety of stellar hosts could retain a CO2- or N2-dominated atmosphere and compare this atmospheric retention distance (ARD) with that of the liquid-water HZ. We combined planetary atmosphere models with stellar evolution models. The atmospheric models produced by the thermochemical Kompot code allowed us to calculate the Jeans escape rates for different stellar masses, rotation rates, and ages. These loss rates allowed us to determine the closest distance a planet is likely to retain a CO2- or N2-dominated atmosphere. Using stellar rotation evolution models, we modelled how these retention distances evolve as the X-ray and ultraviolet activity of the star evolves. We find that the overlap of the HZ and the ARD occurs earlier around slowly rotating stars. Additionally, we find that HZ planets orbiting stars with masses under 0.4 M_\odot are unlikely to retain any atmosphere, due to the lower spin-down rate of these fully convective stars. We also show that the initial rotation rate of the star can impact the likelihood of a planet retaining an atmosphere, as an initially fast-rotating star maintains high levels of short-wavelength irradiance for much longer. The orbits of all Earth-like rocky exoplanets observed by JWST in cycles 1 and 2, including HZ planets, fall outside the ARD. Our results will have implications for future target selections of small exoplanet observing programmes with JWST or future instruments such as the Ariel space mission.