What is phosphine (PH₃)?
Phosphine (or PH₃) is a pyramid-shaped molecule with a single phosphorus atom on top, and a base formed of three hydrogen atoms. It is the simplest gas that has phosphorus on it; the phosphorus equivalent of ammonia (for nitrogen), methane (for carbon) and water (for oxygen).
Why haven’t you heard of it?
It’s not popular. Most people haven’t heard of it, and those who have hate it. On Earth it is quite rare, and when it is used it is usually for something rather evil (e.g., war, low-grade meth, insecticides).
On an oxygen-poor planet, phosphine might be produced in large quantities by its rich, anaerobic biosphere.
Who likes it?
Phosphine is toxic to life that uses oxygen metabolism (e.g., you and me). But anaerobic life, living in swamps, and marshlands, and in the intestines of most animals, has no problem with phosphine. Even on Earth, where oxygen is (almost) everywhere, anoxic life produces phosphine in large quantities.
Why is phosphine special?
There are many molecules associated with life, such as methane, water or oxygen. If we detect these molecules on a habitable planet, it could mean we found life. Or not. These molecules have false positives: geological or photochemical processes that can produce them without the intervention of life. Phosphine does not. I found that, if detected on a rocky planet, phosphine can only mean life.
Phosphine and me.
I have been researching phosphine for a little over a decade. I simulated how phosphine interacts with light (it interacts very well), wrote my PhD thesis on it (not a page turner), investigated its presence on Earth (with much help from William and Janusz), looked at its potential as a biosignature (it has a lot of potential as a biosignature), and then I did a photoshoot with it.