Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopy (for example, refs. 1,2) provides the necessary means by constraining the abundances of oxygen-

The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atmospheric properties using transmission spectroscopy 1-4. However, these efforts have been hampered by modelling degeneracies between composition and cloud properties that are caused by limited data quality 5-9. Here we present the transmission spectrum

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems 1,2. Access to the chemical inventory of an exoplanet requires high-precision observations, often inferred from individual molecular detections with low-resolution space-based 3-5

Transmission spectroscopy 1-3 of exoplanets has revealed signatures of water vapour, aerosols and alkali metals in a few dozen exoplanet atmospheres 4,5. However, these previous inferences with the Hubble and Spitzer Space Telescopes were hindered by the observations' relatively narrow wavelength range and spectral resolving power, which precluded

Carbon dioxide (CO 2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO 2 is an indicator of the metal enrichment (that is, elements heavier than helium, also called `metallicity') 1-3, and thus the formation processes of the primary atmospheres of hot gas giants 4-6. It is also one