Abstract
Y dwarfs are the coldest class of brown dwarfs and have similar effective temperatures to temperate exoplanets, with the coldest Y dwarfs having 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. JWST NIRSpec observations of these objects are revolutionizing the characterization of these worlds with high SNR, moderate resolution spectra. 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.
