There could be seven times as many potentially habitable planets as previously believed, Scottish scientists have calculated.
Previous calculations of the so-called Goldilicks zone - the region around a star where life could exist - have been based on whether liquid water could be present on the surface.
But, says the team, this isn't the whole picture. On Earth, there are vast areas under the surface which can and do support life, and this is likely to be the case elsewhere.
"Life ‘as we know it’ requires liquid water. Traditionally, planets have been considered ‘habitable’ if they are in the ‘Goldilocks zone’. They need to be not too close to their sun but also not too far away for liquid water to persist, rather than boiling or freezing, on the surface," says Sean McMahon from the University of Aberdeen’s School of Geosciences.
"However, we now know that many micro-organisms — perhaps half of all living things on Earth — reside deep in the rocky crust of the planet, not on the surface."
While planet surfaces tend to be warmed by their suns, heat also comes from a planet's interior. Crust temperature increases with depth, so that planets that are too cold for liquid water on the surface may still be warm enough underground to support life. This could even be the case on 'rogue' planets floating in interstellar space.
"We have developed a new model to show how ‘Goldilocks zones’ can be calculated for underground water and hence life," says McMahon. "Our model shows that habitable planets could be much more widespread than previously thought."