Can life exist outside of the habitable zone?

The notion of a habitable zone in astrobiology is looking ever more shaky. According to the definition on Nasa’s Exoplanet Exploration site, this zone of a solar system is ‘The distance from a star at which liquid water could exist on orbiting planets’ surfaces.’ That implies the zone is an annulus with a maximum and minimum radius, which is undermined even in our own solar system by the fact that several worlds, notably Jupiter’s moon Europa and Saturn’s moon Enceladus, lie outside the conventional habitable zone but have water below their icy crusts kept liquid by tidal heating. Enceladus’s ocean was recently shown to contain phosphates, completing the inventory of elements needed for life as we know it.

But perhaps habitable zones are both bigger and smaller than we once thought. Astrophysicist Cassandra Hall and colleagues at the University of Georgia, US, have argued that exoplanet searches for signs of life might be more narrowly focused on the ‘photosynthetic habitable zone’ where both liquid water and oxygen-producing photosynthesis can occur. They identify five exoplanets seen by the Kepler space telescope that fit this criterion, making them prime targets for spectroscopic examination of their atmospheres.