Phosphine Depletion in Brown Dwarf Atmospheres Explained due to Metal Phosphide Formation

Abstract Phosphine (PH 3 ) is predicted to be the dominant phosphorus-bearing gas in cool substellar atmospheres, yet observations have revealed a puzzling pattern. While Jupiter and Saturn exhibit measurable PH 3 abundances sustained by vertical mixing, most brown dwarfs show severe phosphine depletion. The recent detection of PH 3 in two metal-poor brown dwarfs, contrasted with its absence in higher-metallicity counterparts, suggests a metallicity-controlled sequestration mechanism. The origin of this discrepancy has remained unresolved. Here, we show that PH 3 is efficiently sequestered in metal-rich brown dwarf atmospheres through the formation of condensed metal phosphides. Through density functional theory calculations and atmospheric chemical equilibrium modeling, we demonstrate that phosphide-forming reactions under brown dwarf temperature–pressure conditions preferentially partition phosphorus into solid phases over gaseous PH 3 . This mechanism can explain the depletion of PH 3 in higher-metallicity brown dwarfs, while its persistence in metal-poor cases reflects the reduced availability of condensing metals. In contrast, in giant planets like Jupiter and Saturn, Fe, Mg, and Ni condense deep below the photosphere into a (dilute) core, largely preventing phosphide formation from depleting atmospheric PH 3 . Our findings reveal that phosphine abundance is strongly coupled to metallicity and atmospheric condensation chemistry, providing a unified framework to interpret its distribution across substellar atmospheres. Finally, future work is needed to assess the impact of nonequilibrium effects such as kinetics.