Brown dwarfs are objects that sit between planets and stars: they are more massive than planets but not heavy enough to sustain stable fusion in their cores. Their interiors are scorching hot, while their outer layers are relatively cool, and this temperature difference drives powerful convective currents — atmospheric mixers, if you will, that continuously stir the material.

Using spectral analysis (splitting light into its constituent colors to identify chemical composition via absorption and emission lines) on the James Webb Space Telescope, scientists have detected phosphine (PH₃) in the atmosphere of the brown dwarf Wolf 1130C — a gas previously found in large amounts only on gas giants like Jupiter. Here, its concentration is ten times higher than anything seen before.

On our planet, this gas is associated with the smell of rotting organic matter, but in the cosmic 'cocktail' it mixes in a vacuum where your nose is useless.

This discovery gives astronomers a new chemical marker to better distinguish giant exoplanets from brown dwarfs: differences in phosphine content may reflect different atmospheric dynamics and internal structures, which is especially important when studying objects whose nature remains ambiguous.