Image caption Scientists should not exclude planets that reside in colder regions

New computer models suggest there could be many more habitable planets out there than previously thought.

Scientists have developed models to help them identify planets in far-away solar systems that are capable of supporting life.

Estimates of habitable planet numbers have been based on the likelihood of them having surface water.

But a new model allows scientists to identify planets with underground water kept liquid by planetary heat.

The research was presented at the British Science Festival in Aberdeen.

Water is fundamental for life as we know it.

Planets too close to their sun lose surface water to the atmosphere through evaporation.

Surface water on planets located in the more frigid distant reaches from their sun is locked away as ice.

The dogma was, for water to exist in its life-giving liquid form, a planet had to be the right distance from its sun - in the habitable zone.

As Sean McMahon, the PhD student from Aberdeen University who is carrying out the work explained: "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.

"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 researchers are starting to think that the Goldilocks theory is far too simple.

Planetary heat •A planet is warmed by two sources of heat - solar energy and internal heat •The further away a planet is from its sun the less energy it receives and surface water freezes •As the distance increases underground water also starts to freeze •But if the planet is large enough and produces enough internal heat, it could still contain deep reservoirs of liquid water capable of supporting life, no matter how far away from the sun

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.

Immense quantities of water in fact - teeming with primitive life.

As Prof John Parnell, also from Aberdeen University 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."

So 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.

Explaining their rationale, Mr McMahon said: "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."

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.

Even a planet so far from the star that it receives almost no solar heat could still maintain underground liquid water.

According to Mr McMahon, "There will be several times more [habitable] planets".