What does this equation mean?

What lets these three very different kinds of evidence be compared at all is a shared logical structure that the Astrobiology community has tried to formalize explicitly rather than leave implicit. David Catling and coauthors proposed treating a biosignature detection as a Bayesian inference problem: a prior probability that a given planet or sample harbors life, updated by the likelihood of the observed data under a “life” hypothesis versus under one or more specific abiotic alternative hypotheses, yields a posterior probability rather than a binary yes-or-no verdict [ 7 ] . Written compactly, for a single candidate abiotic alternative $H_{ab}$ competing with a life hypothesis $H_{bio}$ given data D : $P(H_{bio} \mid D) = \frac{P(D \mid H_{bio})\, P(H_{bio})}{P(D \mid H_{bio})\, P(H_{bio}) + P(D \mid H_{ab})\, P(H_{ab})}$. The formula is not a calculator anyone runs to get a number; the value of stating it is that it forces every claim to name its competing abiotic hypothesis explicitly rather than treating the absence of an alternative explanation as evidence that none exists. A biosignature claim is exactly as strong as the list of abiotic pathways it has priced into P(D $\mid$ $H_{ab}$) and no stronger — a point equally true of a rover’s organic detection, a spectroscopic retrieval, and a bench synthesis claiming relevance to a real planet.