Phosphorus-bearing molecules PO and PN at the edge of the Galaxy

Despite its importance in planet formation and biology¹, phosphorus has been identified only in the inner 12 kpc of the Galaxy^2-19. The study of this element has been hindered in part by unfavourable atomic transitions^2,4,20. Phosphorus is thought to be created by neutron capture on ²⁹Si and ³⁰Si in massive stars^20,21, and released into the interstellar medium by Type II supernova explosions^2,22. However, models of galactic chemical evolution must arbitrarily increase the supernovae production²³ to match observed abundances. Here we present the detection of gas-phase phosphorus in the Outer Galaxy through millimetre spectra of PO and PN. Rotational lines of these molecules were observed in the dense cloud WB89-621, located 22.6 kpc from the Galactic Centre²⁴. The abundances of PO and PN in WB89-621 are comparable to values near the Solar System²⁵. Supernovae are not present in the Outer Galaxy²⁶, suggesting another source of phosphorus, such as 'Galactic Fountains', where supernova material is redistributed through the halo and circumgalactic medium²⁷. However, fountain-enriched clouds are not found at such large distances. Any extragalactic source, such as the Magellanic Clouds, is unlikely to be metal rich²⁸. Phosphorus instead may be produced by neutron-capture processes in lower mass asymptotic giant branch stars²⁹ which are present in the Outer Galaxy. Asymptotic giant branch stars also produce carbon²¹, flattening the extrapolated metallicity gradient and accounting for the high abundances of C-containing molecules in WB89-621.