A Biologically Defined Habitable Phase Space for Exoplanets

The concept of habitability in exoplanet science is still largely defined by astrophysical boundary conditions, particularly the hypothetical presence of surface liquid water within a circumstellar habitable zone. Most approaches rarely incorporate quantitative biological constraints. Here, we present a biologically grounded approach to exoplanet habitability based on the measured growth limits of terrestrial microorganisms, and show how integrating these data can refine and extend current habitability concepts.

Building on a large-scale meta-analysis of cultured prokaryotic microorganisms, we define a multidimensional phase space of habitability describing the combinations conditions that allow sustained microbial growth, rather than only survival. Long-term biosphere persistence and detectability through biosignature production require active metabolism and replication, while survival alone is only relevant under transient or episodic conditions. Within this framework, habitability is evaluated across key conditions, such as temperature, radiation environment, oxygen availability, pH, and salinity. Using growth-based limits, we build empirical habitable ranges that can be mapped onto planetary surface and subsurface environments predicted by exoplanet climate and interior models. The growth temperature ranges were also used to define biologically-constrained habitable zones.

Finally, we outline how these empirically derived biotic habitable zones can be combined with stellar, planetary, and atmospheric models to define revised habitability criteria for exoplanets, moving from water-based to life-informed approaches. This work provides a quantitative bridge between microbiology and exoplanet science, and a step toward biologically realistic assessments of habitable worlds.