Astronomers using the James Webb Space Telescope have detected the molecule phosphine beyond our Solar System, according to a new study, finding it in the atmosphere of the cold brown dwarf Wolf 1130C. The presence of the phosphorus-containing molecule phosphine (PH₃) is well established in the atmospheres of Jupiter and Saturn, as well as in some giant stars. Although models predict that PH₃ should be similarly common in gas giant exoplanets and brown dwarfs, searches for the molecule in the atmospheres of those objects have shown it to be in very low abundance, if not totally absent. Using the JWST NIRSpec instrument, Adam Burgasser and colleagues obtained high-resolution spectra of the cold, metal-poor brown dwarf Wolf 1130C. Within these observations, Burgasser et al. identified multiple absorption lines of PH₃, indicating an abundance of 0.100 ± 0.009 parts per million – an amount comparable to Jupiter and Saturn and far higher than previous limits for other exoplanets or brown dwarfs. Comparison with atmospheric models shows the properties can be reproduced if there is strong vertical mixing of an atmosphere with low abundances of elements heavier than helium. “The inability of models to consistently explain all these sources indicates an incomplete understanding of phosphorus chemistry in low-temperature atmospheres,” write the authors. “We therefore caution against the use of phosphine as a biosignature until these discrepancies are resolved.”