A Terminology and Quantitative Framework for Assessing the Habitability of Solar System and Extraterrestrial Worlds

The search for extraterrestrial life in the solar system and beyond is a key science driver in astrobiology, planetary science, and astrophysics. A critical step is the identification and characterization of potential habitats, both to guide the search and to interpret its results. However, a well-accepted, self-consistent, flexible, and quantitative terminology and method of assessment of habitability are lacking. Our paper fills this gap based on a 3 yr long study by the NExSS Quantitative Habitability Science Working Group. We review past studies of habitability but find that the lack of a universally valid definition of life prohibits a universally applicable definition of habitability. A more nuanced approach is needed. We introduce a quantitative habitability assessment framework that enables self-consistent, probabilistic assessment of the compatibility of two models: first, a habitat model, which describes the probability distributions of key conditions in the habitat, and second, a viability model, which describes the probability that a metabolism is viable given a set of environmental conditions. We provide an open-source implementation of this framework and four examples as a proof of concept: (a) comparison of two exoplanets for observational target prioritization, (b) interpretation of atmospheric O₂ detection in two exoplanets, (c) subsurface habitability of Mars, and (d) ocean habitability in Europa. These examples demonstrate that our framework can self-consistently inform astrobiology research over a broad range of questions. The proposed framework is modular so that future work can expand the range and complexity of models available, both for habitats and for metabolisms.