Phosphine has caused quite a stir in the astronomical world lately. That was largely due to its (still hotly debated) detection in the atmosphere of Venus. While the only known way for phosphine to be created on terrestrial worlds, like Venus, is through some sort of biological origin, it is relatively common among larger gas giants and even “brown dwarfs” - failed stars larger than Jupiter but not quite large enough to start their own hydrogen fusion process. Previously, we hadn’t yet seen phosphine in the atmosphere of brown dwarf in other solar systems, but a new paper from a diverse group of researchers, available in pre-print form on arXiv, used data collected by the James Webb Space Telescope (JWST) to find it for the first time. They also realized the mechanism that made it so hard to spot in the first place - the object’s metallicity.
Metallicity is a very common concept in astronomy, but is counterintuitive to what could be thought of as the common use of the word. In chemistry “metals” are defined chemical elements with very specific characteristics. However, in astronomy, a star’s (or failed star’s) metallicity is defined as what amount of elements other than hydrogen and helium are present in it.
Very old stars have lower “metallicities”, as the process for forming elements higher on the periodic table than helium involves a previous generation of star exploding in a supernovae. So, typically at least, the older a star is, the lower its metallicity. Our own Sun has relatively high metallicity, but there are some stars and brown dwarfs in the “thick disk” of the galaxy that are much older with lower levels of metallicity.
Fraser discusses why phosphorous is so important to astrobiology.The researcher team used the NIRSpec instrument on the JWST to observe one of those brown dwarfs in the thick disk - Wolf 1130C. When they looked at its spectral profile, there was a clear absorption signal centered around 4.3um - right where phosphine is expected. So why hadn’t it been detected before around other, similar objects?
Jupiter and Saturn have abundant phosphine - in fact their phosphorous content is estimated to be 5-16 times the level found in our already metal-rich Sun. We can see the signal for phosphine clearly due to the fact that a confounding factor isn’t present in their upper atmospheres - carbon dioxide. CO2 has extremely strong absorption lines at the same point in the spectrum that phosphine does, and can easily overwhelm the smaller signal attributed to the less-abundant compound. In Jupiter and Saturn, the upper atmosphere isn’t very warm, so most of the carbon present in it is tied up in methane (CH4) rather than CO2. Methane has a different spectral signature, and therefore doesn’t interfere with the phosphine absorption in the way that carbon dioxide does.
However, for brown dwarfs such as Wolf 1130C, which is estimated to be 44 times the size of Jupiter, their upper atmospheres are much warmer, due in part because there is some amount of fusion, usually of deuterium, going on in their core. This increased temperature allows for the formation of carbon dioxide - at least in stars with high metallicity. The signal for phosphine was so clear on Wolf 1130C because, given its low metallicity, it had a tiny amount of carbon dioxide compared to its peers. In essence, it’s not that phosphine isn’t present in brown dwarfs, it's that the signal showing it has been overwhelmed by a much stronger signal of a more common element.
Fraser and Pamela discuss "failed stars" as brown dwarfs are sometimes known.The researchers went a step farther, and proved that the phosphine wasn’t just given to Wolf 1130C from one of the two companion stars in its three star system. They confirmed that it was created in the brown dwarf itself, and moved up to the outer atmosphere, where it can be detected. It also means that other, low metallicity brown dwarfs should have the same phosphine signatures - a theory that can be tested with further observations.
This has obvious implications for finding phosphine on other worlds. Though no one is claiming that phosphine on a gas giant or brown dwarf is anything other than purely chemical in nature, the fact that the absorption line for this compound is so closely tied to that of a much more common compound (CO2) that isn’t a biosignature can make its use as one much more difficult. The fact that Venus has plenty of carbon dioxide in its atmosphere also complicates the previous findings further. As researchers continue to push for finding new and better biosignatures, this research on phosphine should help temper their expectations and make them look again at the data to ensure they’re seeing what they think they’re seeing.
Learn More:
A. J. Burgasser - Observation of undepleted phosphine in the atmosphere of a low-temperature brown dwarf
UT - Metals Are Critical To Life - We Should Screen Exoplanets For Them
UT - Astronomers Think They've Found a Reliable Biosignature. But There's a Catch
UT - Photochemistry and Climate Modeling of Earth-like Exoplanets