Scientists have detected the molecule phosphine, a compound potentially linked to the presence of life, in the atmosphere of the brown dwarf star WOLF 1130c using the James Webb Space Telescope.
Previous observations have detected phosphine in the atmospheres of other brown dwarf stars and giant planets such as Jupiter and Saturn. However, the concentration levels in those discoveries were significantly lower than previous models had predicted.
Conversely, the levels detected around WOLF 1130c, which is located 54 light-years from the Sun in the constellation Cygnus, were nearly a perfect match. The new research was led by Professor Adam Burgasser from the University of California, San Diego’s (UCSD) Department of Astronomy and Astrophysics.
“It’s basically spot on with what models have long predicted,” Burgasser told The Debrief.
The professor, who is the director of UCSD’s Cool Star Lab, added that detecting phosphine would not normally make for a “science-level result” if previous detections had also matched models. However, finding phosphine levels that match predictions “muddies the waters even further,” since the models work some of the time but not others, indicating our understanding of phosphorus chemistry is clearly incomplete.”
“Our challenge now is to understand why!” Burgasser said.
Phosphine detections have previously made headlines due to their connection to biological processes on Earth. For example, researchers detected phosphine in the clouds of Venus in 2020, 2021, and 2023, conditions that may be hospitable for certain anaerobic life forms on Earth. Professor Burgasser noted that because phosphine levels are continuously depleted by natural processes, they must be renewed in the atmosphere by some other process, “which on Earth is biology-based.”
“For terrestrial worlds which are dominated by O2/N2 (Earth) or CO2 (Venus, Mars), phosphorus preferentially binds to oxygen in various forms of phosphate,” he explained. “Phosphine (PH3) is rare in these atmospheres and has to be replenished by active processes.”
In the atmospheres of planets like Jupiter and Saturn, as well as brown dwarf stars, which are hydrogen-based, phosphorus binds to hydrogen first, naturally replenishing phosphine levels. So, detecting phosphine around a rocky exoplanet could be an indicator of biological processes, whereas phosphine in the atmosphere of a brown dwarf is predicted by naturally occurring processes.
“This is what distinguishes a biosignature – it shouldn’t form naturally in an atmosphere (of a rocky planet), implying life is the source,” Burgasser told The Debrief. “Phosphine fits the bill in oxygen-rich atmospheres but not in hydrogen-rich atmospheres.”
Still, the predicted concentrations haven’t always agreed with observation, making the new detection even more complicated.
“Prior to JWST, phosphine was expected to be abundant in exoplanet and brown dwarf atmospheres, following theoretical predictions based on the turbulent mixing we know exists in these sources,” said co-author Sam Beiler, in a statement announcing the team’s discovery.
One example described by Professor Burgasser as a “heroic analysis by Melanie Rowland and collaborators,” found phosphine in the atmosphere of a brown dwarf star with the JWST. However, in that case, it was 100x less than the models had predicted.
“This is why our article title refers to ‘undepeleted’ phosphine,” the professor told The Debrief, “as this is the first time we’ve actually agreed with the models!”
Beiler agreed, noting that every observation the team obtained with JWST “has challenged the theoretical predictions — that is, until we observed Wolf 1130C.”
Assistant Professor of Astronomy at San Francisco State University, Eileen Gonzales, also a co-author on the study, described the process she used to confirm the levels.
“To determine the abundances of molecules in Wolf 1130C, I used a modeling technique known as atmospheric retrievals,” Professor Gonzales explained. “This technique uses the JWST data to back out how much of each molecular gas species should be in the atmosphere.”
“It’s like reverse engineering a really delicious cookie when the chef wouldn’t give up the recipe,” she added.
One proposed explanation for the difference involves the unique material makeup of WOLF 1130c. Part of a triple star system that includes a red dwarf (1130a) and a white dwarf (1130b), the star has an abundance of metals in its atmosphere that could affect phosphine renewal.
“It may be that in normal conditions phosphorus is bound up in another molecule such as phosphorus trioxide,” Beiler explained. “In the metal-depleted atmosphere of Wolf 1130C, there isn’t enough oxygen to take up the phosphorus, allowing phosphine to form from the abundant hydrogen.”
Although the detection is unlikely to be an indicator of life, Burgasser told The Debrief that in this experiment, brown dwarf stars can serve as a “control sample” for understanding phosphine production and replenishment in a non-biological setting. He also said the result is a clear indicator that scientists “need to do more work to understand the chemistry of phosphine.”
“Our result shows that using biosignatures like phosphine in our search for life beyond Earth requires us to really understand the chemistry of this molecule in its natural (non-life) environment,” he said. “Until we get these atmospheres right, we should remain skeptical about using phosphine as a signature for life.”
The study “Observation of undepleted phosphine in the atmosphere of a low-temperature brown dwarf” was published in Science.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.