Phosphine (PH₃) has recently been identified (but only tentatively) in the circumstellar shell of IRC +10216 by Agúndez et al, but there is also evidence that is a constituent of the atmospheres of both Jupiter and Saturn. These early detections have been confirmed by later observations, including measurements made by the Composite Infrared Spectrometer (CIRS) on the Cassini probe.

Phosphine is the next-heavier periodic cousin of ammonia. Like ammonia, phosphine also has a pungent aroma, but it is toxic as well.