Protosolar D-to-H abundance and one part per billion PH3 in the coldest brown dwarf
We detected deuterium (in the form of deuterated methane) in an extrasolar atmosphere for the first time. We also detected phosphine at the one part-per-billion level. We quantify the effective mixing timescale for PH3 and calculate the D/H ratio for WISE 0855, the coldest brown dwarf.