Cause of phosphine depletion in brown dwarf and exoplanet atmospheres

Determine the physical and chemical mechanisms responsible for the low observed abundance of phosphine (PH3) in brown dwarf and giant exoplanet atmospheres, in contrast to the higher PH3 abundances predicted by disequilibrium vertical mixing models and observed in Jupiter, Saturn, and the brown dwarf Wolf 1130C.

Background

The paper reports a robust detection of phosphine (PH3) in the atmosphere of the metal-poor brown dwarf Wolf 1130C, with an abundance consistent with disequilibrium vertical mixing models. This finding contrasts with prior observations of late T and Y dwarfs and giant exoplanets, where PH3 is strongly depleted relative to model predictions.

The authors explore several possible explanations, including condensation chemistry (e.g., NH4H2PO4) and uncertainties in intermediate phosphorus oxide thermodynamics (e.g., P4O6), but these do not consistently account for the observed variations: they can explain depletion seen elsewhere but not the higher PH3 seen in Wolf 1130C and in Jupiter and Saturn. Consequently, the underlying cause of widespread PH3 depletion in other cool atmospheres remains unresolved.