The CO-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. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (around 89) in the Solar System, but do vary on galactic scales with a C/C isotopologue ratio of around 68 in the current local interstellar medium. In molecular clouds and protoplanetary disks, CO/CO ratios can be altered by ice and gas partitioning, low-temperature isotopic ion-exchange reactions and isotope-selective photodissociation. Here we report observations of CO 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 CO/CO ratio of [Formula: see text](90% confidence), a substantial enrichment in C 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 C through fractionation.