Constraints on the possible atmospheres on TRAPPIST-1 b: insights from 3D climate modeling

¹ Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, 98 bis bd Arago, 75014 Paris, France
² Laboratoire de Météorologie Dynamique/IPSL, CNRS, Sorbonne Université, Ecole Normale Supérieure, Université PSL, Ecole Polytechnique, Institut Polytechnique de Paris, 75005 Paris, France
³ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁴ AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif-sur-Yvette, France
⁵ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Centre pour la Vie dans l’Univers, Université de Genève, Geneva, Switzerland
⁸ Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290 Sauverny, Switzerland
⁹ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁰ Integrated Space Science and Technology Institute, Department of Physics, American University, Washington, DC, USA
¹¹ Astrobiology Research Unit, Université de Liège, Allée du 6 août 19, Liège, 4000, Belgium

^★ Corresponding author.

Received: 24 February 2025
Accepted: 9 June 2025

Abstract

Context. JWST observations of the secondary eclipse of TRAPPIST-1 b at 12.8 and 15 µm revealed a very bright dayside. These measurements are consistent with an absence of atmosphere. Previous 1D atmospheric modeling also excludes – at first sight – CO₂-rich atmospheres. However, only a subset of the possible atmosphere types has been explored, and ruled out, to date. Recently, a full thermal phase curve of the planet at 15 µm with JWST has also been observed, allowing for more information on the thermal structure of the planet.

Aims. We first looked for atmospheres capable of producing a dayside emission compatible with secondary eclipse observations. We then tried to determine which of these are compatible with the observed thermal phase curve.

Methods. We used a 1D radiative-convective model and a 3D global climate model (GCM) to simulate a wide range of atmospheric compositions and surface pressures. We then produced observables from these simulations and compared them to available emission observations.

Results. We found several families of atmospheres compatible at 2σ with the eclipse observations. Among them, some feature a flat phase curve and can be ruled out with the observation, and some produce a phase curve still compatible with the data (i.e., thin N₂ –CO₂ atmospheres, and CO₂ atmospheres rich in hazes). We also highlight different 3D effects that could not be predicted from 1D studies (redistribution efficiency, atmospheric collapse).

Conclusions. The available observations of TRAPPIST-1 b are consistent with an airless planet, which is the most likely scenario. A second possibility is a thin CO₂-poor residual atmosphere. However, our study shows that different atmospheric scenarios can result in a high eclipse depth at 15 µm. It may therefore be hazardous, in general, to conclude on the presence of an atmosphere from a single photometric point.