The idea of habitability was introduced by Dole (10, 11) to refer to those planetary conditions suitable for human life. The word has since come to imply requirements both less stringent and less anthropocentric, referring instead to the stability of liquid water at a world's surface. A circumstellar habitable zone is the volume of space around a single or multiple-star system within which an Earth-like world could support surface liquid water (12, 13).
The historical emphasis on surface liquid water is easy to understand. First, life on Earthstill our sole example of a biologyutterly depends on liquid water (7, 14). Second, primary production of organic matter is dominated by sunlight-driven photosynthesis at Earth's surface (15). In the traditional view, a planet's mass must be large enough to maintain sufficient geological activity to power the climate-stabilizing carbonate-silicate feedback cycle (16). For surface liquid water to persist longer than 1 Gyr, a planetary mass greater than 0.1 Earth masses seems required, by analogy to Mars (12). Similar constraints have been derived for satellites of giant planets (17).
Europa's putative subsurface ocean suggests that the traditional view of planetary habitability should be broadened (7, 11, 18). This suggestion is strengthened by the elucidation of the terrestrial subsurface biosphere (19), the microbial biomass of which appears comparable to Earth's entire surface biomass, although subsurface biological turnover times are long (20). If some terrestrial life exists or could exist independently of surface photosynthesis, then the possibilities for extraterrestrial biospheres greatly expand. If life originated on Mars during its apparent early clement period (21), it is possible that its progeny remain in subsurface hydrothermal niches (22).
A more fundamental question is whether life can originate at depth, independently of the sun. If not, then only worlds that have clement surfaces (Earth) or that once did (Mars) could host endemic biologies, although interplanetary transfer of microorganisms might still introduce life to previously sterile worlds (23). But if the origin of life could occur at depth, then worlds like Europa could host their own biologies. Processes at hydrothermal vents may have been important in Earth's origin of life (24, 25), but it remains unclear whether the entire origin of life could have been independent of sunlight-driven surface conditions and photochemistry.