To calculate habitable zones across the galaxy we take into account a host star’s luminosity, along with the planet’s distance from it and that planet’s size relative to the star.

The well-established Goldilocks theory, however, fails to take life beneath the surface into account, where temperatures dramatically change.

“As you get deeper below a planet’s surface, the temperature increases, and once you get down to a temperature where liquid water can exist – life can exist there too,” PhD student at the University of Aberdeen Sean McMahon said. “The deepest known life on Earth is 5.3km below the surface, but there may well be life even 10km deep in places on Earth that haven’t yet been drilled.”

The computer model was used to estimate what the temperature beneath the surface would be of any given planet it had the necessary parameters for. It found that the habitable zone would be around three times bigger than previously thought if it included the first 5km beneath an Earth-like planet’s surface. When depths of up to 10km below the Earth’s surface were included, the model found the habitable zone was 14 times wider. Applied to our own Solar System, it means the habitable zone extends beyond Saturn.

“The results suggest life may occur much more commonly deep within planets and moons than on their surfaces.”

“Rocky planets a few times larger than the Earth could support liquid water at about 5km below the surface even in interstellar space (i.e. very far away from a star), even if they have no atmosphere because the larger the planet, the more heat they generate internally.”

Study: seemingly cool planets could be warm enough to host life underground (Wired UK)