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On the Earth, phosphine is a colorless and highly toxic gas that is produced by microorganisms that do not need oxygen to function. So I just wonder why the scientists decided to look for it in the planets.
First, let's start by a direct quote from the paper
"PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life." https://www.nature.com/articles/s41550-020-1174-4
That's a bit different from the exclusive focus on biological production that the press widely publicized.
As to why someone looked for it, see this https://ui.adsabs.harvard.edu/abs/2020AsBio..20..235S/abstract (full text: https://arxiv.org/ftp/arxiv/papers/1910/1910.05224.pdf)
1) we know of no non-biological process, on rocky planets such as Earth, that could produce phosphine continuously. Continuously because it is constantly degraded by UV (and O, OH, H).
2) it is _relatively_ easy to detect.
From the paper "Phosphine is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets from any source that could generate the high fluxes required for detection. "
And yes, "no known" could go both ways.
During the next 10 years or so, we should be able to get spectroscopy results from the atmosphere of at least some exo-planets. It makes sense to worry now about what we should look for when the instrumental capability is there.