A Terminology and Quantitative Framework for Assessing the Habitability of Solar System and Extraterrestrial Worlds
Our paper fills this gap based on a three year-long study by the NExSS Quantitative Habitability Science Working Group. We reviewed 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 (QHF) 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. 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.
Visual summary of key considerations and arguments about the nature and scope of the definition and use of the term “habitable”. There are strong and valid arguments both for a simpler and more specific meaning and for a more complex and more universal meaning. Any single definition for “habitability” fails to meet the majority of the requirements. — astro-ph.EP
Illustration of the basis of the Framework for Habitability: The comparison of the environmental conditions predicted by the habitat model and the environmental conditions required by the metabolism model. — astro-ph.EP
Overview of the NExSS Framework for Habitability. The framework assesses the suitability of habitats for model organisms/ecosystems. The potential habitats are described through a combination of system/planet-specific data and priors informed by exoplanet statistics. Habitat suitability is assessed by applying a function specific to species/organisms to the habitat properties. The outcomes are probabilistic in nature. This framework is both flexible and specific. A special case of habitat suitability is included that corresponds to “surface liquid habitability”. — astro-ph.EP
Overview of the temperatures ranges in which representative types of terrestrial organisms are biologically active (i.e., likely to complete life cycles). — astro-ph.EP
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.
Daniel Apai, Rory Barnes, Matthew M. Murphy, Tim Lichtenberg, Noah Tuchow, Regis Ferriere, Kevin Wagner, Antonin Affholder, Renu Malhotra, Baptiste Journaux, Allona Vazan, Ramses Ramirez, Abel Mendez, Stephen R. Kane, Veronica H. Klawender, NExSS Quantitative Habitability Science Working Group
Comments: In press, Planetary Science Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2505.22808 [astro-ph.EP] (or arXiv:2505.22808v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.22808
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Submission history
From: Daniel Apai Dr
[v1] Wed, 28 May 2025 19:33:51 UTC (6,369 KB)
https://arxiv.org/abs/2505.22808
Astrobiology,
