Protosolar D/H abundance in the coldest brown dwarf

Y dwarfs are the coldest class of brown dwarfs and have similar effective temperatures to temperate exoplanets. The coldest Y dwarfs actually have the same masses as giant planets. We can therefore use their atmospheres as proxies for planetary atmospheres, testing our understanding of physics and chemistry for these complex, cool worlds. At these cold temperatures, their atmospheres are cold enough for water clouds to form, and chemistry slows down, pushing us further from chemical equilibrium than in warmer classes of planets. JWST NIRSpec observations of these objects are revolutionizing the characterization of these worlds with high SNR, moderate resolution spectra. These have been used to measure the abundances of abundant species like water, methane, and ammonia; species that trace chemical reactions like carbon monoxide and phosphine; and even isotopologues of ammonia. Here, we will show atmospheric retrieval results using public GTO data of the coldest known Y dwarf, WISE 0855 (using NIRSpec G395M spectra), which has an effective temperature of ~250 K. We will present a detection of deuterium in an atmosphere outside of the solar system via a relative measurement of deuterated methane (CH3D) and standard methane. From this, we infer the D/H ratio of a substellar object outside the solar system for the first time. We discuss our interpretation of these results and the implications for brown dwarf and giant exoplanet formation and evolution.