Identification of carbon dioxide in an exoplanet atmosphere

Abstract

Carbon dioxide (CO₂) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO₂ is an indicator of the metal enrichment (i.e., elements heavier than helium, also called “metallicity”)^1-3, and thus formation processes of the primary atmospheres of hot gas giants^4-6. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets^7-9. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO₂, but have not yielded definitive detections due to the lack of unambiguous spectroscopic identification^10-12. Here we present the detection of CO₂ in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science Program (ERS)^13,14. The data used in this study span 3.0 - 5.5 µm in wavelength and show a prominent CO₂ absorption feature at 4.3 µm (26σ significance). The overall spectrum is well matched by one-dimensional, 10x solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide, and hydrogen sulfide in addition to CO₂, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 µm that is not reproduced by these models.