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 Galaxy2-19 . The study of this element has been hindered in part by unfavourable atomic transitions2,4,20 . Phosphorus is thought to be created by neutron capture on 29 Si and 30 Si in massive stars20,21 , and released into the interstellar medium by Type II supernova explosions2,22 . 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.