Detection of phosphine in a brown dwarf atmosphere raises more questions

Phosphorus is one of six key elements necessary for life on Earth. When combined with hydrogen, phosphorus forms the molecule phosphine (PH₃), an explosive, highly toxic gas.

Found in the atmospheres of the gas giant planets Jupiter and Saturn, phosphine has long been recognized as a possible biosignature for anaerobic life, as there are few natural sources of this gas in the atmospheres of terrestrial planets. On Earth, phosphine is a byproduct of decaying organic swamp matter.

Now a team of researchers, led by University of California San Diego Professor of Astronomy and Astrophysics Adam Burgasser, has reported the detection of phosphine in the atmosphere of a cool, ancient brown dwarf named Wolf 1130C. Their work appears in Science.

Phosphine was detected in Wolf 1130C’s atmosphere using observations obtained with the James Webb Space Telescope (JWST), the first telescope with the sensitivity to look at these celestial objects in detail. The mystery, however, is not why phosphine was found, but why it’s missing in other brown dwarf and gas giant exoplanet atmospheres.

“Our astronomy program, called Arcana of the Ancients, focuses on old, metal-poor brown dwarfs as a means of testing our understanding of atmospheric chemistry,” said lead author Burgasser. “Understanding the problem with phosphine was one of our first goals.”

In the hydrogen-rich atmospheres of gas giant planets like Jupiter and Saturn, phosphine forms naturally. As such, scientists have long predicted that phosphine should be present in the atmospheres of gas giants orbiting other stars, and in their more massive cousins, brown dwarfs—objects sometimes called “failed stars” because they do not fuse hydrogen.

Yet phosphine has largely eluded detection, even in prior JWST observations, suggesting problems with our understanding of phosphorus chemistry.

“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, who graduated from the University of Toledo and is now postdoctoral scholar at Trinity College Dublin.

Beiler, who has led previous work studying the lack of phosphine in brown dwarfs, stated, “Every observation we’ve obtained with JWST has challenged the theoretical predictions—that is until we observed Wolf 1130C.”

In the star system Wolf 1130ABC, located 54 light-years from the sun in the constellation Cygnus, the brown dwarf Wolf 1130C follows a wide orbit around a tight double star system, composed of a cool red star (Wolf 1130A) and a massive white dwarf (Wolf 1130B).

Wolf 1130C has been a favorite source for brown dwarf astronomers due to its low abundance of “metals”—essentially any elements other than hydrogen and helium—compared to the sun.

Unlike other brown dwarfs, the team easily spotted phosphine in JWST’s infrared spectral data of Wolf 1130C. To fully understand the implications of their findings, they needed to quantify the abundance of this gas in Wolf 1130C’s atmosphere. This was done by Assistant Professor of Astronomy at San Francisco State University Eileen Gonzales, also a co-author on the study.

“To determine the abundances of molecules in Wolf 1130C, I used a modeling technique known as atmospheric retrievals,” explained Gonzales.

“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.”

Gonzales’s models showed that abundant phosphine was the secret ingredient in Wolf 1130C. Specifically, she found that phosphine was present at the predicted theoretical abundances of about 100 parts per billion.

While the researchers are delighted by their discovery, it raises an issue: why is phosphine present in the atmosphere of this brown dwarf and not others?

One possibility is the low abundance of metals in Wolf 1130C’s atmosphere, which may change its underlying chemistry. “It may be that in normal conditions, phosphorus is bound up in another molecule such as phosphorus trioxide,” explained Beiler.

“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.”

The team hopes to explore this possibility with new JWST observations that will search for phosphine in the atmospheres of other metal-poor brown dwarfs.

Another possibility is that phosphorus was generated locally in the Wolf 1130ABC system, specifically by its white dwarf, Wolf 1130B.

“A white dwarf is the leftover husk of a star that has finished fusing its hydrogen,” explained Burgasser. “They are so dense that when they accrete material on their surface they can undergo runaway nuclear reactions, which we detect as novae.”

While astronomers haven’t seen evidence of such events in the Wolf 1130ABC system in recent history, novae typically have outburst cycles of thousands to tens of thousands of years. This system has been known for just over a century, and early, unseen outbursts could have left a legacy of phosphorus pollution.

Earlier studies have proposed that a significant fraction of phosphorus in the Milky Way could have been synthesized by this process.

Understanding why this one brown dwarf shows a clear signature of phosphine may lead to new insights into the synthesis of phosphorus in the Milky Way and its chemistry in planetary atmospheres.

Burgasser explains, “Understanding phosphine chemistry in the atmospheres of brown dwarfs, where we don’t expect life, is crucial if we hope to use this molecule in the search for life on terrestrial worlds beyond our solar system.”