Biosignatures, phosphine, and the brown-dwarf twist
In the search for life beyond Earth, scientists look for biosignatures—molecules whose presence could signal biology. Phosphine is one such candidate because on our planet it is produced primarily by anaerobic organisms and is rarely formed through abiotic processes in oxygen-rich environments. Yet before scientists declare any molecule a conclusive sign of life, they must understand how it could arise in atmospheres that are very different from Earth’s.
Now, for the first time, researchers have identified appreciable amounts of phosphine in the atmosphere of a brown dwarf, a substellar object that straddles the line between planets and stars. The finding, reported in Science 2025 (DOI: 10.1126/science.adu0401), suggests that phosphine can be produced by nonbiological chemistry in environments far removed from planetary surfaces. The lead author, Adam Burgasser, a professor of astronomy and astrophysics at UC San Diego, describes brown dwarfs as “failed stars” because they are too light to sustain hydrogen fusion yet more massive than giant planets.
The result challenges assumptions that phosphine, if detected, must point toward life. While Earth’s phosphine is linked to microbial activity, models predicting phosphine in brown-dwarf atmospheres imply that abiotic chemical processes could generate detectable amounts. In the past, observations of phosphine in such atmospheres have fallen short by two orders of magnitude or more, leaving researchers cautious about interpreting any future detections as biosignatures.
The observation: JWST spots 100 ppb phosphine
Using the James Webb Space Telescope, Burgasser’s team obtained a spectrum of Wolf 1130C, a brown dwarf about 54 light-years from Earth and part of a three-body system. Phosphine emits infrared radiation in a spectral window that JWST can probe with unprecedented precision, a range that is largely inaccessible to the Hubble Space Telescope and many ground-based facilities. From the spectrum, Eileen Gonzales and colleagues estimated an atmospheric phosphine abundance of about 100 parts per billion (ppb).
Though the number may seem small, Burgasser emphasizes that this is “the first time we’ve seen phosphine at the level we had expected,” marking a milestone for atmospheric chemistry in substellar objects. The result provides a rare data point that helps calibrate models of how phosphorus compounds form and persist in cool, dusty atmospheres where temperatures and pressures diverge sharply from those on planets or stars.
What makes this detection robust?
Several factors contribute to the robustness of the finding. First, the phosphine signal falls within a strong infrared band accessible to JWST, enabling a clean spectral fingerprint amid the complex chemistry of a brown-dwarf atmosphere. Second, the team’s analysis cross-checked multiple spectral features and nonlinear atmospheric models to rule out alternative explanations. Finally, the calculated abundance aligns with the level at which researchers warned phosphine could plausibly exist in these environments, helping bridge theory with observation.
Why this matters for astrobiology
Brown dwarfs are not expected to be habitable in the traditional sense, but the discovery matters for how scientists interpret biosignatures elsewhere. If abiotic processes can produce detectable phosphine in brown-dwarf atmospheres, scientists must carefully disentangle potential biosignatures from purely chemical origins when evaluating exoplanet and brown-dwarf atmospheres. This finding prompts a broader reexamination of phosphorus chemistry under a wider range of temperatures, pressures, and metallicities, and it encourages the development of complementary biosignature indicators that can help distinguish biology from chemistry.
What comes next
The study opens several avenues for future work. Astronomers will aim to replicate and extend measurements to Wolf 1130C and other brown dwarfs, building a statistically meaningful picture of phosphine across substellar atmospheres. Theoretical chemists will refine models of abiotic phosphine production, exploring pathways that could operate in cool, cloud-rich environments. Observationally, JWST and successor facilities can search for correlated signals from related phosphorus-bearing molecules, helping to clarify whether phosphine’s presence is a rare quirk or a common feature in substellar atmospheres. In the broader context of exoplanet science, this result strengthens the case for multi-molecule, multi-wavelength approaches to biosignatures, ensuring that life signals are interpreted with an eye to the rich diversity of planetary and substellar chemistry.
Conclusion
The detection of phosphine in a brown dwarf atmosphere marks a watershed moment for the field. It demonstrates that abiotic processes can generate detectable phosphine in environments far removed from Earth and underscores the careful nuance required when using phosphine as a biosignature. As observations accumulate and models improve, scientists will be better equipped to distinguish true signs of life from the diverse chemistry of the cosmos.
