JWST: Identification of carbon dioxide in an exoplanet atmosphere

Summary: Carbon dioxide (CO2) is a key chemical species found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of metal enrichment (i.e. in elements heavier than helium, also called "metallicity"), and therefore of the processes of formation of primary atmospheres hot gas giants. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets. Previous photometric measurements of transiting planets with the Spitzer Space Telescope gave clues to the presence of CO2 but did not yield definitive detections due to the lack of unambiguous spectroscopic identification. 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). The data used in this study covers a wavelength of 3.0 to 5.5 {\mu}m and shows a prominent CO2 absorption feature at 4.3 {\mu}m (significance of 26{\ sigma}). The global spectrum fits well with one-dimensional 10x solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should contain water, carbon monoxide and hydrogen sulphide in addition to CO2, but little methane. Additionally, we also tentatively detect a small absorption feature near 4.0 {\mu}m which is not reproduced by these models.