2025MNRAS.542.3402K

The phosphorus budget of planets is intertwined with their formation history and is thought to influence their habitability. The chemical reservoirs and volatile versus refractory budget of phosphorus in planet-forming environments have so far eluded empirical characterization. We employ high-resolution spectra from Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) in the ultraviolet and the PI230 receiver on the single-dish APEX telescope in the sub-mm to constrain the phosphorus budget in the well-characterized HD 100546 star and protoplanetary disc system. We measure $\log {(P/H)_{\star }}=-7.50^{+0.23}_{-0.28}$ on the stellar surface, which traces the total inventory of P in accreting gas and dust from the inner disc. The inner disc gas, inside of the main dust trap, has $\log {(P/H)_{\rm in}}\lesssim -8.70$, and the outer disc gas $\log {(P/H)_{\rm out}}\lesssim -9.30$. Phosphorus in the disc is carried by a relatively refractory reservoir, consistent with minerals such as apatite or schreibersite, or with ammonium phosphate salts, in terms of sublimation temperature. We discuss the impact this might have on the two protoplanets around HD 100546. Our results contribute to our understanding of the chemical habitability of planetary systems and lay a foundation for future explorations, especially in the context of JWST and Ariel which can study phosphorus in exoplanet atmospheres.