JWST Measurements of ¹³C, ¹⁸O, and ¹⁷O in the Atmosphere of Super-Jupiter VHS 1256 b

Isotope ratios have recently been measured in the atmospheres of directly imaged and transiting exoplanets from ground-based observations. The arrival of JWST allows us to characterize exoplanetary atmospheres in further detail and opens up wavelengths inaccessible from the ground. In this work we constrain the carbon and oxygen isotopes C-13, O-18, and O-17 from CO in the atmosphere of the directly imaged companion VHS 1256 b through retrievals of the similar to 4.1-5.3 mu m NIRSpec G395H/F290LP observations from the early-release science program (ERS 1386). We detect and constrain (CO)-C-13-O-16, (CO)-C-12-O-18, and (12)C17O at 32 sigma, 16 sigma, and 10 sigma confidence respectively, thanks to the very high signal-to-noise observations. We find the ratio of abundances are more precisely constrained than their absolute values, with C-12/C-13 = 62(-2)(+2) , in between previous measurements for companions 2 2 (similar to 30) and isolated brown dwarfs (similar to 100). The oxygen isotope ratios are( 16)O/ O-18 = 425(-28)(+33 )and O-16/ (17 ) = 1010(-100)(+120). Al of the ratios are lower than the local interstellar medium and solar system, suggesting that abundances of the more minor isotopes are enhanced compared to the primary. This could be driven by isotope fractionation in protoplanetary disks, which can potentially alter the carbon and oxygen ratios through isotope selective photodissociation, gas/ice partitioning, and isotopic exchange reactions. In addition to CO, we constrain (H2O)-H-1-O-16 and (CO2)-C-12-O-16 (the primary isotopologues of both species), but find only upper limits on (CH4)-C-12-H-1 and (NH3)-N-14-H-1. This work highlights the power of JWST to constrain isotopes in exoplanet atmospheres, with great promise in determining formation histories in the future. 120 100 . All of the ratios are lower than the local interstellar medium and solar system, suggesting