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The James Webb Space Telescope (JWST) has found phosphine within the ambiance of a brown dwarf — the identical chemical that stoked controversy following claims that it had been detected on Venus and might be coming from life.
This new detection on a brown dwarf is predicted by fashions that simulate alien atmospheres and is a reminder that phosphine shouldn’t be essentially a biosignature. However, astronomers stay puzzled about why some objects include phosphine and others don’t, though concept says it must be there.
The phosphine was recognized within the chilly ambiance of a brown dwarf referred to as Wolf 1130C, which exists in a triple system together with a low-mass red dwarf star and a white dwarf. The phosphine exists with an abundance of 0.1 parts per million, which matches what models of the atmosphere of gas giant planets and brown dwarfs predict. Indeed, both Jupiter and Saturn contain a similar abundance of phosphine to Wolf 1130C.
The problem has been that many brown dwarfs that are expected to show detectable abundances of phosphine do not, and scientists don’t know why.
Phosphine is a phosphorus-based molecule, composed of one atom of phosphorus and three hydrogen atoms. It is also pretty unstable in atmospheric conditions, and chemical reactions can easily break phosphine molecules apart. We see phosphine in Jupiter and Saturn’s clouds because it is formed deep within the hot interiors of the giant planets, and then convection currents carry the phosphine to higher altitudes faster than the rate at which it is destroyed.
This is one of the reasons why the claimed detection of phosphine on Venus is so controversial.
It was in 2020 that a team led by Jane Greaves of the University of Cardiff in Wales detected phosphine in Venus’ atmosphere using the James Clerk Maxwell Telescope in Hawaii and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. On Earth, phosphine occurs naturally as a product of biological processes, and Greaves’ team strongly pushed the biological angle to explain their discovery, leading to speculation that there could be microbes living in Venus’ toxic clouds.
However, a large section of the astronomical community differed with the team’s findings, arguing that there were flaws in the analysis, and other groups have struggled to replicate the findings. In spite of this, Greaves’ team has doubled down on their conclusions, and the presence of phosphine on Venus remains fiercely debated and controversial.
Part of scientists’ disagreement with the discovery is that they find it hard to see how the phosphine could survive in Venus’ atmosphere.
Nevertheless, phosphine is still considered a potential biosignature by astrobiologists in their search for alien life.
However, its existence in the clouds of Jupiter and Saturn, and now on Wolf 1130C, is a reminder that non-biological chemical processes can also produce phosphine. The question is why Jupiter, Saturn and Wolf 1130C have detectable levels of phosphine while other brown dwarfs that have been studied by JWST do not, or at least are so depleted in it that the molecule is not detectable.
There are several possible explanations. One is unique to the Wolf 1130 system. Before it evolved into a white dwarf, Wolf 1130B was a large star with a mass between six and eight times that of our sun. Such stars aren’t quite massive enough to go supernova, so they end their lives much in the same way as our sun will — by expanding to become a red giant and then puffing off their outer layers to form a planetary nebula, while leaving behind their inert core as a white dwarf the size of Earth which, in the case of Wolf 1130B, packs in 1.24 solar masses.
Stars in the six-to-eight solar mass range can produce significant amounts of phosphorus in the latter stages of their life, which they can then belch out into space as the red giant shrugs off its outer layers. If this phosphorus-rich material was spewed all over Wolf 1130C, then it would explain where the phosphorus to form phosphine came from.
It’s a nice theory, but unfortunately it doesn’t pass muster. The white dwarf in the Wolf 1130 system forms a close binary with the low-mass star, Wolf 1130A, while the brown dwarf orbits the pair of them from a distance. A and B are so close that they are tidally locked to each other, meaning they show each other the same face constantly. Their relationship is even more involved than that — the gravitational pull of the white dwarf is actually stretching Wolf 1130A into an egg shape toward it.
When the star that formed the white dwarf died, the outer layers of the red giant would have engulfed Wolf 1130A. If the death of the star that became the white dwarf had deposited phosphorus onto the brown dwarf, then we’d also expect to see an over-abundance of phosphorus on Wolf 1130A, but we do not.
Another possibility is that the presence of phosphine is somehow related to the intrinsic chemical composition of the brown dwarf. Some models predict that atmospheres that contain very few elements heavier than helium have a preponderance for more phosphine. Indeed, 1130C does appear to have a very low abundance of these heavier elements, which astronomers refer to collectively as “metals.” Similarly, Jupiter and Saturn also have low “metallicities.”
The exact reason why a lack of heavy elements promotes phosphine is multifaceted: Not only does it help create the conditions in which phosphine can form and survive longer than it normally would, but the relative lack of other molecules present in the atmosphere means that there is less to interfere with the phosphine signal in the brown dwarf’s spectrum, causing it to stand out more.
The problem is that other brown dwarfs observed by JWST also have low metallicities, but they don’t show the expected amounts of phosphine.
These ambiguities prompt the authors of the new research, led by Adam Burgasser of the University of California, San Diego, to question how useful a biosignature phosphine is when we cannot even say for sure how it forms on distant planets and brown dwarfs.
“The inability of models to consistently explain all these sources indicates an incomplete understanding of phosphorus chemistry in low-temperature atmospheres,” the authors said. “We therefore caution against the use of phosphine as a biosignature until these discrepancies are resolved.”
If nothing else, the new study reminds us that, even if the detection of phosphine on Venus turns out to be real, its origin could very well be abiotic rather than biologically related. It’s not time to get excited about life on any of these worlds just yet.
The findings were published on Oct. 2 in the journal Science.
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