Abstract EANA2026-84

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. However, we know long-term biosphere persistence and potential detectability of produced biosignatures require active metabolism and replication, while survival alone is only relevant under transient or episodic conditions. Therefore,  exoplanet habitability approaches should incorporate quantitative biological constraints. 

Here, we present a biologically grounded approach to exoplanet habitability based on the growth amplitudes of cultured Bacteria and Archaea, and show how integrating these data can refine and extend current habitability concepts. After gathering a comprehensive dataset of 26,142 cultured prokaryotes, we analysed microbial growth conditions for temperature, pH, and salinity, together with physiological traits relevant to exoplanets, particularly oxygen tolerance. Using growth-based limits across these environmental parameters, we build an empirical habitable range, or phase space of habitability, that can be mapped onto planetary surface and subsurface environments predicted by exoplanet climate and interior models. 

Principal component analysis (PCA) showed that microbial growth niches are influenced primarily by a combination of temperature and salinity gradient and secondarily by pH. Given the dominance of anoxic atmospheres of potentially habitable exoplanets, we compared the growth conditions and environmental niches of aerobic and anaerobic organisms. Oxygen tolerance classes occupy distinct regions of this phase-space, with obligate anaerobes extending toward hotter, saltier, and more acidic conditions, while obligate aerobes cluster closer to moderate environments, as well as dominating cold environments. Among cultured prokaryotes with oxygen tolerance data (11,022 species), 63% are obligate aerobes and 37% are facultative or obligate anaerobes. 

We then applied this framework to three potentially habitable exoplanets: GJ 581 c, TRAPPIST-1 e, and LHS 1140 b. We used existing surface temperature distributions predicted through general climate models (GCMs) to get estimated environmental conditions for these planets. Then, we analyze our dataset to identify microorganisms compatible with growth under the selected planetary scenarios. For the hot, likely tidally locked super-Earth GJ 581 c, where dayside temperatures may be mostly > 50 ºC, we identify several anaerobic organisms capable of thermophilic growth, for instance the chemolithoautotrophic archaeum Thermoproteus tenax and the fermentative bacteria Fervidobacterium pennivorans, which could be able to occupy this exoplanet’s dayside niches. For TRAPPIST-1 e, whose predicted climate depends strongly on atmospheric and surface assumptions, but may have an average dayside temperature < 0 ºC, we filtered for organisms capable of subzero growth and found that the best cold-adapted candidates are aerobic, though extreme anaerobic methanogens like Methanogenium frigidum could serve as chemolithoautotrophic primary producers in such environments. For LHS 1140 b, a potential ocean world, we focus on organisms adapted to oligotrophic conditions, reflecting the possible nutrient-cycling limitations of ocean planets, and on species with tolerance to higher pressures, relevant to scenarios involving a deep global ocean and/or a denser atmosphere. 

This work provides a quantitative bridge between microbiology and exoplanet science, and a step toward biologically realistic assessments of habitable worlds.