You had to know that if we were going to discover alien life in 2020, it would be because of the toxic gas it's spewing out.
This is actually a great test for the idea of detecting life by biomarker (the only possible way we're going to detect life on exoplanets). It's relatively straightforward to send a probe to scoop up some atmosphere and analyze it.
On the other hand, as a chemist who has only read summaries so far, I've always been skeptical of the "we can't think of any other way to get this chemical besides life." There are always ways to spit out weird molecules when you have access to precursors and full spectrum sunlight.
Agreed. Phosphine is a very basic molecule. It's like methane, except with phosphorus instead of carbon. There are various ways it can come about, and one in particular which would work on Venus.
To start with, you need a phosphoric mineral (plenty in the volcanic surface rock) and sulfuric acid (plenty of that in the atmosphere). The sulfuric acid, heat, and phosphoric mineral (probably something like hydroxyapatite or something) would create phosphoric acid and other byproducts. However, phosphoric acid breaks down into phosphine at temperatures greater than 200C, which certainly is not a problem on the surface of Venus.
Phosphine is less dense than the CO2 which makes up 95% of the atmosphere, so it eventually rises through the atmosphere. The opacity of the atmosphere helps preserve the phosphine (breaks down in UV light, much like methane) until it makes it high enough in the atmosphere.
I'll need to go read the paper. It doesn't make a whole lot of sense to claim this is biomarker when all the materials are there for it to come about naturally.