Phosphorus monoxide is an unstable radical inorganic compound with molecular formula PO. Phosphorus monoxide is notable as one of the few molecular compounds containing phosphorus that has been detected outside of Earth. Other phosphorus containing molecules found in space include PN, PC, PC2, HCP and PH3. It was detected in the circumstellar shell of VY Canis Majoris and in the star forming region catalogued as . The compound has been found to have been initially produced in star-forming regions, and speculated to be carried by interstellar comets throughout outer space, including to the early Earth.