  Isotope abundance ratios play an important role in astronomy and planetary
sciences, providing insights in 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 (~89) in the Solar System, but do vary on galactic scales with
12C/13C~68 in the current local interstellar medium. In molecular clouds and
protoplanetary disks, 12CO/13CO isotopologue ratios can be altered by ice and
gas partitioning, low-temperature isotopic ion exchange reactions, and
isotope-selective photodissociation. Here we report on the detection of 13CO in
the atmosphere of the young, accreting giant planet TYC 8998-760-1 b at a
statistical significance of >6 sigma. Marginalizing over the planet's
atmospheric temperature structure, chemical composition, and spectral
calibration uncertainties, suggests a 12CO/13CO ratio of 31 [+17,-10] (90%
confidence), a significant enrichment in 13C with respect to the terrestrial
standard and the local interstellar value. Since the current location of TYC
8998 b at >160 au is far beyond the CO snowline, we postulate that it accreted
a significant fraction of its carbon from ices enriched in 13C through
fractionation. Future isotopologue measurements in exoplanet atmospheres can
provide unique constraints on where, when and how planets are formed.
