There is a possibility of many more habitable planets in the universe than previously thought, according to a new computer model that allows scientists to identify the liquid underground water.
Estimates of habitable planet numbers have been based on the likelihood of them having surface water.
It is believed that for water to exist in its life-giving liquid form, a planet had to be at the right distance from its sun -- in the habitable zone.
Now a new model allows scientists to identify planets with underground water kept liquid by planetary heat, the 'BBC News' reported.
"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," said Sean McMahon, researcher from Aberdeen University.
"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," he said.
But researchers are starting to think that the Goldilocks theory is far too simple.
Planets can receive two sources of heat -- heat direct from the star and heat generated deep inside the planet.
As you descend through the crust of the Earth, the temperature gets higher and higher. Even when the surface is frozen, water can exist below ground.
Prof John Parnell, also from Aberdeen University, who is leading the study, said: "There is a significant habitat for microorganisms below the surface of the Earth, extending down several kilometres. And some workers believe that the bulk of life on Earth could even reside in this deep biosphere."
The Aberdeen team are developing models to predict which far-flung planets might harbour underground reservoirs of liquid water with the possibility of alien life.
"If you take into account the possibility of deep biospheres, then you have a problem reconciling that with the idea of a narrow habitable zone defined only by conditions at the surface," McMahon said while explaining their rationale.
As you move away from the star the amount of heat a planet receives from the star decreases and the surface water freezes -- but any water held deep inside will stay liquid if the internal heat is high enough - and that water could support life, the researchers said.
Even a planet so far from the star that it receives almost no solar heat could still maintain underground liquid water.
"There will be several times more (habitable) planets," McMahon concluded. The research was presented at the British Science Festival in Aberdeen.