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.
Yin et al. (2026) studied this question.