The ¹³CO-rich atmosphere of a young accreting super-Jupiter

Isotope abundance ratios have an important role in astronomy and planetary sciences, providing insights into the origin and evolution of the Solar System, interstellar chemistry and stellar nucleosynthesis^1,2. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (around 89) in the Solar System^1,3, but do vary on galactic scales with a ¹²C/¹³C isotopologue ratio of around 68 in the current local interstellar medium^4-6. In molecular clouds and protoplanetary disks, ¹²CO/¹³CO ratios can be altered by ice and gas partitioning⁷, low-temperature isotopic ion-exchange reactions⁸ and isotope-selective photodissociation⁹. Here we report observations of ¹³CO in the atmosphere of the young, accreting super-Jupiter TYC 8998-760-1 b, at a statistical significance of more than six sigma. Marginalizing over the planet's atmospheric temperature structure, chemical composition and spectral calibration uncertainties suggests a ¹²CO/¹³CO ratio of [Formula: see text](90% confidence), a substantial enrichment in ¹³C with respect to the terrestrial standard and the local interstellar value. As the current location of TYC 8998-760-1 b at greater than or equal to 160 astronomical units is far beyond the CO snowline, we postulate that it accreted a substantial fraction of its carbon from ices enriched in ¹³C through fractionation.