Phosphine has been making a big splash in the astronomical world lately. This was mainly due to its (still hotly debated) detection in the atmosphere of Venus. While the only known route for the creation of phosphine on terrestrial worlds like Venus is biological in origin, it is relatively common on larger gas giants and even “brown dwarfs” – failed stars larger than Jupiter but not quite large enough to start their own hydrogen fusion process. We hadn’t previously seen phosphine in the atmosphere of brown dwarfs in other solar systems, but a new paper from a diverse group of researchers, available in preprint form on arXiv, used data collected by the James Webb Space Telescope (JWST) for the first time to find it. They also recognized the mechanism that made the object so difficult to detect in the first place – the metallicity of the object.
Metallicity is a very common concept in astronomy, but it contradicts what one might think of as the general usage of the word. In chemistry, “metals” are defined chemical elements with very specific properties. However, in astronomy, the metallicity of a star (or failed star) is defined as the amount of elements other than hydrogen and helium present in it.
Very old stars have lower “metallicity” because the process of forming elements higher than helium on the periodic table requires the explosion of an earlier generation of stars in a supernova. Normally, the older a star is, the lower its metallicity. Our own Sun has a relatively high metallicity, but there are some stars and brown dwarfs in the galaxy’s “thick disk” that are much older and have lower metallicity.
Fraser discusses why phosphorus is so important to astrobiology.
The research team used the NIRSpec instrument at JWST to observe one of these brown dwarfs in the thick disk – Wolf 1130C. When they looked at the spectral profile, there was a clear absorption signal centered around 4.3 µm – exactly where phosphine is expected. So why hasn’t it been discovered near other, similar objects before?
Jupiter and Saturn have plenty of phosphine – in fact, their phosphorus content is estimated to be 5 to 16 times that of our already metal-rich Sun. We can clearly see the signal for phosphine because there is no interfering factor in their upper atmospheres – carbon dioxide. CO2 has extremely strong absorption lines at the same point in the spectrum as phosphine and can easily mask the smaller signal attributed to the less abundant compound. In Jupiter and Saturn, the upper atmosphere is not very warm, so most of the carbon in it is bound in methane (CH4) rather than CO2. Methane has a different spectral signature and therefore does not interfere with phosphine absorption like carbon dioxide.
However, for brown dwarfs like Wolf 1130C, which is estimated to be 44 times larger than Jupiter, the upper atmosphere is much warmer, in part because some fusion, mostly deuterium, is occurring in its core. This increased temperature enables the formation of carbon dioxide – at least in stars with high metallicity. The signal for phosphine was so clear on Wolf 1130C because, due to its low metallicity, it contained only a small amount of carbon dioxide compared to its counterparts. Essentially, it’s not that phosphine isn’t present in brown dwarfs, but rather that the signal indicating it was overshadowed by a much stronger signal from a more abundant element.
Fraser and Pamela discuss “failed stars,” as brown dwarfs are sometimes called.
The researchers went a step further and proved that the phosphine was delivered to Wolf 1130C by more than just one of the two companion stars in its three-star system. They confirmed that it originated in the brown dwarf itself and entered the outer atmosphere, where it can be detected. This also means that other brown dwarfs with low metallicity should have the same phosphine signatures – a theory that can be tested with further observations.
This has obvious implications for the search for phosphine on other worlds. Although 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 of this compound is so closely tied to that of a much more common compound (CO2) that is not a biosignature can make its use as such significantly difficult. The fact that Venus has plenty of carbon dioxide in its atmosphere also complicates previous findings. As researchers continue to push forward in the search for new and better biosignatures, this research on phosphine should help temper their expectations and make them take another look at the data to make sure they’re seeing what they think they’re seeing.
Learn more:
AJ Burgasser – Observation of undepleted phosphine in the atmosphere of a low-temperature brown dwarf
UT – Metals are vital – we should study exoplanets for them
UT – Astronomers believe they have found a reliable biosignature. But there’s a catch
UT – Photochemistry and climate modeling of Earth-like exoplanets
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