An artist rendition released by the European Space Agency on Wednesday, Nov. 28, 2007 shows the main bodies of the solar system, the Sun, Mercury, Venus, the Earth, from left in foreground, Uranus, Neptune, Saturn, Jupiter and Mars, from left in background. The Moon, the Earth's natural satellite, is seen at right in foreground, as the relative size of the orbits of the planets is not respected. Nearby planet Venus is looking a bit more Earth-like with frequent bursts of lightning confirmed by a new European space probe. For nearly three decades, astronomers have said Venus probably had lightning, ever since a 1978 NASA probe showed signs of electrical activity in its atmosphere. But experts were not sure because of signal interference. (AP Photo/ESA/HO) ** MAGAZINES OUT - NO SALES **

London - New computer models suggest that there are many more habitable planets in distant solar-systems than previously thought.

Research from scientists at the University of Aberdeen was presented at the British Science Festival on Monday showing the new models that help identify planets with a greater likelihood of having underground water, and therefore able to support life.

Previous estimates of planets capable of supporting life had been based on the likelihood of there being surface water available. It was thought that for water to exist in liquid form, a planet must be a certain distance from its sun, in a “habitable” or “Goldilocks” zone.

“Life as we know it requires liquid water. Traditionally, planets have been considered habitable if they are in the Goldilocks zone,” said Sean McMahon of the University of Aberdeen's School of Geosciences.

He added: “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. However, we now know that many micro-organisms reside deep in the rocky crust of the planet, not on the surface. There will be several times more [habitable] planets.”

The new research is based on estimates that even when the surface of a planet is frozen, huge quantities of water, teeming with life, could exist below, warmed by heat generated within the planet and not its star.

“There is a significant habitat for micro-organisms below the surface of the Earth, extending down several kilometres,” said Professor John Parnell of Aberdeen University. “And some believe that the bulk of life on Earth could reside in this deep biosphere.”

The university is developing models to predict which planets might harbour underground reservoirs of liquid water with the possibility of alien life.

Suns warm planet surfaces but heat also comes from planet interiors. Crust temperature increases with depth so planets that are too cold for liquid water on the surface may be sufficiently warm underground to support life.

Mr McMahon said: “We have developed a new model to show how 'Goldilocks zones' can be calculated for underground water and hence life. Our model shows that habitable planets could be much more widespread than previously thought.” - The Independent