Phosphorus is one among six essential elements that make life on Earth possible. When it bonds with hydrogen, it forms phosphine (PH3), a highly toxic and explosive gas. This compound is present in the atmospheres of the gas giants Jupiter and Saturn and has long been viewed as a possible biosignature for anaerobic life. On Earth, phosphine arises naturally from decaying organic material in swamps, but on other planets, it’s rare and intriguing.
A research team led by University of California San Diego Professor of Astronomy and Astrophysics Adam Burgasser has now detected phosphine within the atmosphere of a cool, ancient brown dwarf referred to as Wolf 1130C. Their findings were recently published in Science.
The team made the invention using the James Webb Space Telescope (JWST), the primary instrument powerful enough to research these dim, low-temperature objects intimately. The surprising part, nevertheless, isn’t 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 method of testing our understanding of atmospheric chemistry,” said Burgasser. “Understanding the issue with phosphine was one among our first goals.”
Under normal conditions, phosphine naturally forms within the hydrogen-rich atmospheres of gas giants like Jupiter and Saturn. Due to that, researchers have long assumed it must also exist in similar environments around other stars, including brown dwarfs, that are sometimes called “failed stars” because they’re too small to fuse hydrogen like true stars.
Yet phosphine has been elusive in previous JWST observations, hinting that something may 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 all 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 statement we have obtained with JWST has challenged the theoretical predictions — that’s until we observed Wolf 1130C.”
The Unusual System of Wolf 1130ABC
Wolf 1130C is an element of a posh three-star system situated 54 light-years away within the constellation Cygnus. The brown dwarf orbits a detailed binary composed of a cool red star (Wolf 1130A) and a dense white dwarf (Wolf 1130B). Astronomers have long been excited by this method because Wolf 1130C accommodates far fewer “metals” (elements heavier than hydrogen and helium) than the Sun, providing a helpful laboratory for studying primitive cosmic chemistry.
Unlike in previous brown dwarf observations, the JWST data revealed a powerful infrared signal from phosphine in Wolf 1130C’s atmosphere. To know how much of the gas was present, the team turned to Assistant Professor Eileen Gonzales from San Francisco State University, who makes a speciality of atmospheric modeling.
“To find out the abundances of molecules in Wolf 1130C, I used a modeling technique referred to as atmospheric retrievals,” said Gonzales. “This system uses the JWST data to back out how much of every molecular gas species ought to be within the atmosphere. It’s like reverse engineering a very delicious cookie when the chef would not surrender the recipe.”
Her evaluation confirmed that phosphine was present in the expected amount — about 100 parts per billion.
Why This Brown Dwarf and Not Others?
The invention raises a brand new query: why does this particular brown dwarf contain phosphine while others don’t? One possibility involves the thing’s unusual chemical composition. “It could be that in normal conditions phosphorus is certain up in one other molecule reminiscent of phosphorus trioxide,” explained Beiler. “Within the metal-depleted atmosphere of Wolf 1130C, there’s not enough oxygen to take up the phosphorus, allowing phosphine to form from the abundant hydrogen.”
The team plans to check this concept through upcoming JWST observations of other metal-poor brown dwarfs to see if the identical pattern appears.
A Clue from a Dying Star
One other hypothesis is that phosphorus can have been produced locally inside 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’re so dense that once they accrete material on their surface they will undergo runaway nuclear reactions, which we detect as novae.”
Although astronomers haven’t observed any nova events in this method in recent history, such outbursts often recur every few thousand years. Because Wolf 1130ABC has been known for less than a couple of century, earlier eruptions could have gone unnoticed, abandoning traces of phosphorus in the encompassing space. Previous research has suggested that lots of the Milky Way’s phosphorus atoms can have originated from these stellar explosions.
Unraveling the Origins of Phosphorus within the Cosmos
Understanding why Wolf 1130C accommodates clear evidence of phosphine could offer essential insights into how phosphorus forms within the galaxy and the way it behaves in planetary atmospheres. As Burgasser explained, “Understanding phosphine chemistry within the atmospheres of brown dwarfs where we do not expect life is crucial if we hope to make use of this molecule within the search for all times 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.