Fabian Gruebel, Karan Molaverdikhani, Barbara Ercolano, Katy L. Chubb, Paola Caselli, Tommaso Grassi, Rosa Arenales-Lope, Dwaipayan Dubey

Hydrogen-isotopologues are commonly used to trace the chemical processing and origin of hydrogen-bearing species throughout the Universe, however, their abundance remains unconstrained for extrasolar planets. Here, we report atmospheric retrieval evidence for the deuterated water molecule HDO in an exoplanet atmosphere, retrieved from James Webb Space Telescope transmission spectra of the hot Jupiter WASP-39 b, resulting in a deuterium-to-hydrogen ratio in water of $4.0^{+1.3}_{-1.1} \times 10^{-3}$. The inferred value is substantially higher than those measured for the Solar System gas giants and overlaps numerically with values reported for some protostellar and inner Solar System environments. This enrichment may reflect either inherited water-rich material accreted beyond the snow line or isotopic processing in the observable atmosphere through transport, photochemistry, and subsequent escape.