Airy worlds or barren rocks? On the survivability of secondary atmospheres around the TRAPPIST-1 planets

Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: gwenael.van.looveren@univie.ac.at

Received: 26 September 2023
Accepted: 11 January 2024

Abstract

Context. The James Webb Space Telescope is currently at the forefront of the search for atmospheres of exoplanets. However, the observation of atmospheres of Earth-like planets pushes the limits of the instruments, and often, multiple observations must be combined. As with most instruments, telescope time is unfortunately extremely limited. Over the course of cycle 1, approximately 100 hours have been dedicated to the TRAPPIST-1 planets. This system is therefore studied in unusually great detail. However, the first two sets of observations of the innermost two planets show that these planets most likely lack a thick atmosphere. The question therefore arises whether terrestrial planets around M stars have atmospheres or do not have atmospheres at all.

Aims. We aim to determine the atmospheric survivability of the TRAPPIST-1 planets by modelling the response of the upper atmosphere to incoming stellar high-energy radiation. Through this case study, we also aim to learn more about rocky planet atmospheres in the habitable zone around low-mass M dwarfs.

Methods. We simulated the upper atmospheres of the TRAPPIST-1 planets using the Kompot code, which is a self-consistent thermo-chemical code. Specifically, we studied the atmospheric mass loss due to Jeans escape induced by stellar high-energy radiation. This was achieved through a grid of models that account for the differences in planetary properties and irradiances of the TRAPPIST-1 planets, as well as different atmospheric properties. This grid allows for the explorations of the different factors influencing atmospheric loss.

Results. The present-day irradiance of the TRAPPIST-1 planets would lead to the loss of an Earth’s atmosphere within just some hundreds of million years. When we take into account the much more active early stages of a low-mass M dwarf, the planets undergo a period of even more extreme mass loss, regardless of planetary mass or atmospheric composition.

Conclusions. The losses calculated in this work indicate that it is unlikely that any significant atmosphere could survive for any extended amount of time around any of the TRAPPIST-1 planets based on present-day irradiance levels. The assumptions used here allow us to generalise the results, and we conclude that the results tentatively indicate that this conclusion applies to all Earth-like planets in the habitable zones of low-mass M dwarfs.