Seventy years after the first identification of an interstellar species, CH, new diatomic molecules are still being discovered. The most recent of these is the observation of phosphorous monoxide (PO) by Tenenbaum and co-workers, as reported in 2007. PO was observed in the envelope of the largest known star, the red hypergiant VY Canis Majoris The observations were made at the Arizona Radio Observatory with their submillimeter telescope.

Phosporus monoxide is a periodic cousin of NO.