Up to now astronomers have estimated the possible numbers of habitable planets in distant solar systems by assuming they were in the "habitable zone" of their systems where surface water might exist that could support life, the BBC reported Monday.
"It's the idea of a range of distances from a star within which the surface of an Earth-like planet is not too hot or too cold for water to be liquid," researcher Sean McMahon, a doctoral student from Aberdeen University, said.
"So traditionally people have said that if a planet is in this Goldilocks zone -- not too hot and not too cold -- then it can have liquid water on its surface and be a habitable planet."
But a new model by Aberdeen researchers allows scientists to identify planets that might be outside the "zone" but could still have underground water kept liquid by planetary heat.
Planets can have two sources of heat, they noted, energy that comes from its star and that generated deep inside the planet.
Beneath Earth's crust, for example, interior heat means that even when the surface is frozen, water -- and life -- can exist underground.
"There is a significant habitat for microorganisms below the surface of the Earth, extending down several kilometers," Aberdeen Professor John Parnell said.