On a brown dwarf dozens of light years from Earth, astronomers have detected trace amounts of phosphine, a molecule that on this planet is produced by living things.
This discovery is not life, the astronomers say. Any life as we know it would be impossible to sustain in such an environment.
But the detection, published in the journal Science on Thursday, may help scientists on their quest to find both phosphine and life elsewhere in the galaxy. The presence of phosphine in the atmosphere of a brown dwarf, an object that falls somewhere between a planet and a star, can refine scientific understanding of how and where the molecule could be found in other places. Eventually, that knowledge will help astronomers determine if phosphine is a reliable hint of life on rocky planets in the Milky Way.
“We have to make sure we do the work of understanding all of the natural processes that can make this molecule, before we can rule them out and say there must be a biological source,” said Adam Burgasser, an astrophysicist at the University of San Diego who led the study.
In other words, scientists must know the chemistry of phosphine, before they can know if it is a result of biology.
Phosphine, a molecule made of three hydrogen atoms and one phosphorus atom, is tricky to create and easy to destroy. On Earth, it is largely made by microbial life in off-putting places, like rotting swamp plants and animal intestines. A widely debated announcement of phosphine on the planet Venus in 2020 spurred interest in the molecule as a possible hint of life on other worlds.
But phosphine is also found naturally in places that are uninhabitable, like the giant gas planets of Jupiter and Saturn. Astronomers reasoned, then, that such worlds outside the solar system should be relatively rich in phosphine, as should brown dwarfs, which have similar atmospheres.
Years of observations with the James Webb Space Telescope, however, fell short. Phosphine was barely there, if at all, leaving astronomers scratching their heads about why the molecule was largely absent. The brown dwarf, named Wolf 1130C, is the only one, so far, that has met expectations.
Dr. Burgasser and his team observed Wolf 1130C by using two instruments on the Webb telescope that measured the wavelengths of light emitted and absorbed by the brown dwarf, information linked to the chemical makeup of its atmosphere. They discovered phosphine on Wolf 1130C at a concentration of 100 parts per billion — that is, for every one billion particles in the atmosphere, 100 of them were phosphine.
“My first impression was shock that we finally saw it,” said Eileen Gonzales, an astrophysicist at San Francisco State University who led the analysis. “We’ve been looking for so long.”
The abundance of phosphine in the brown dwarf’s atmosphere is what scientists had expected to find, according to Melanie Rowland, an astronomer at the American Museum of Natural History who was not involved in the work, but who previously detected phosphine on a different brown dwarf at a much lower concentration. The new discovery is both exciting and frustrating, Dr. Rowland said, because Wolf 1130C isn’t a standard brown dwarf. It is very old, and heavier elements, like oxygen, are largely missing from its atmosphere.
Those peculiarities make it “even more surprising” that the amount of phosphine matches the predictions of astronomers, she said.
It is unclear why other brown dwarfs appear to be lacking in phosphine. Other, more abundant molecules might be hiding its signal. Phosphorus could be condensing and raining out of the sky altogether or atmospheric models of the phosphine’s abundance could be wrong, and Wolf 1130C is somehow unique.
“We just don’t know the chemistry well enough,” Dr. Burgasser said.
Phosphine is uncommon in the universe: The molecule is destroyed when exposed to ultraviolet light, and won’t form in many environments. And yet, the finicky nature of phosphine is exactly what makes it a promising candidate for being a sign of life, according to Clara Sousa-Silva, an astrochemist at Bard College. She was involved with the team that potentially detected phosphine on Venus, and also with the team that discovered the compound in the clouds of the brown dwarf.
Finding the molecule in places that cannot harbor life, Dr. Sousa-Silva said, “will be a critical piece of the puzzle for figuring out what business phosphine has anywhere else, including in a potentially habitable environment.”