Astronomers shocked by mysterious gas found in deep space

A research team led by University of California San Diego Professor of Astronomy and Astrophysics Adam Burgasser has now detected phosphine in the atmosphere of a cool, ancient brown dwarf known as Wolf 1130C. Their findings were recently published in Science.

The team made the discovery using the James Webb Space Telescope (JWST), the first instrument powerful enough to analyze these dim, low-temperature objects in detail. The surprising part, however, is not that phosphine was found -- but that it appears absent from other brown dwarfs and gas giant exoplanets where scientists expected it.

Probing the Chemistry of Ancient Stars

"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 Burgasser. "Understanding the problem with phosphine was one of our first goals."

Under normal conditions, phosphine naturally forms in the hydrogen-rich atmospheres of gas giants like Jupiter and Saturn. Because of that, researchers have long assumed it should also exist in similar environments around other stars, including brown dwarfs, which are sometimes called "failed stars" because they are too small to fuse hydrogen like true stars.

Yet phosphine has been elusive in previous JWST observations, hinting that something might be missing in 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," explained co-author Sam Beiler, a postdoctoral scholar at Trinity College Dublin who recently earned his doctorate from the University of Toledo.

Beiler, who has led earlier studies investigating this very absence, added, "Every observation we've obtained with JWST has challenged the theoretical predictions -- that is until we observed Wolf 1130C."

The Unusual System of Wolf 1130ABC

Wolf 1130C is part of a complex three-star system located 54 light-years away in the constellation Cygnus. The brown dwarf orbits a close binary composed of a cool red star (Wolf 1130A) and a dense white dwarf (Wolf 1130B). Astronomers have long been interested in this system because Wolf 1130C contains far fewer "metals" (elements heavier than hydrogen and helium) than the Sun, providing a valuable laboratory for studying primitive cosmic chemistry.

Unlike in previous brown dwarf observations, the JWST data revealed a strong infrared signal from phosphine in Wolf 1130C's atmosphere. To understand how much of the gas was present, the team turned to Assistant Professor Eileen Gonzales from San Francisco State University, who specializes in atmospheric modeling.

"To determine the abundances of molecules in Wolf 1130C, I used a modeling technique known as atmospheric retrievals," said 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."

Her analysis confirmed that phosphine was present in the predicted amount -- about 100 parts per billion.

Why This Brown Dwarf and Not Others?

The discovery raises a new question: why does this particular brown dwarf contain phosphine while others do not? One possibility involves the object's unusual chemical composition. "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 plans to test this idea through upcoming JWST observations of other metal-poor brown dwarfs to see if the same pattern appears.

A Clue from a Dying Star

Another hypothesis is that phosphorus may have been produced locally within 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," said Burgasser. "They are so dense that when they accrete material on their surface they can undergo runaway nuclear reactions, which we detect as novae."

Although astronomers have not observed any nova events in this system in recent history, such outbursts often recur every few thousand years. Because Wolf 1130ABC has been known for only about a century, earlier eruptions could have gone unnoticed, leaving behind traces of phosphorus in the surrounding space. Previous research has suggested that many of the Milky Way's phosphorus atoms may have originated from these stellar explosions.

Unraveling the Origins of Phosphorus in the Cosmos

Understanding why Wolf 1130C contains clear evidence of phosphine could offer important insights into how phosphorus forms in the galaxy and how it behaves in planetary atmospheres. As Burgasser explained, "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."

This work was supported by NASA/STScI (NAS 5-03127 and AR-2232) and the Heising-Simons Foundation.