Habitable Zone and Atmosphere Retention Distance (HaZARD)

Stellar-evolution-dependent loss models of secondary atmospheres

¹ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
² International Institute for Applied Systems Analysis (IIASA), Schloßplatz 1, 2361 Laxenburg, Austria

^★ Corresponding author; gwenael.van.looveren@univie.ac.at

Received: 14 November 2024
Accepted: 31 January 2025

Abstract

Context. Thanks to the James Webb Space Telescope (JWST), observations of the secondary atmospheres of rocky planets have become possible. Of particular interest are rocky planets orbiting low-mass stars within the habitable zone (HZ). However, no thick secondary atmospheres have been found around Earth-sized planets to date. This leaves open the question of whether secondary atmospheres are rare around Earth-sized rocky exoplanets.

Aims. 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.

Methods. 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.

Results. 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_⊙ 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.

Conclusions. 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.