Mutually Sustaining, Not Fixed
A syllabus entry, which starts from one quotation and pushes on it.
"A planet is not habitable simply because it possesses certain fixed properties; it is habitable because the complex relationships between its geological, atmospheric, chemical, and biological systems are mutually sustaining over geological timescales."
— Charles H. Langmuir and Wally Broecker, How to Build a Habitable Planet, quoted in the Planetarity Syllabus, Habitability Part II
Liquid water, the right distance from the right star, an atmosphere: the checklist version of habitability, and the one astrobiology spent decades refining before admitting its limits. Langmuir and Broecker's sentence quietly throws the checklist out. Habitability isn't a set of boxes a planet ticks once. It's a relationship between systems that has to keep holding, continuously, or it stops being true.
That's the sentence I've built this whole site to answer. A checklist can be inspected once and filed. A relationship that has to keep holding needs something that checks on a schedule, notices when it lapses, and knows what happens next. The Ministry for the Distant Future and Metabolic Standing are two different guesses of mine at what that something looks like.