Underground water could greatly increase the chances of planets harbouring extraterrestrial life, scientists believe.
Taking into account sub-surface habitats broadens the range of places life-sustaining planets can occupy, a study has found.
It means there could be around seven times more life-bearing planets among the stars than is currently believed.
Potentially habitable rocky planets are said to lie in the "Goldilocks zone" - the narrow orbital path that is not too close to their star and not too far away, but "just right" to support surface liquid water.
Several such worlds have already been discovered.
But including the possibility of life below the ground means the Goldilocks zone can be wider, thereby increasing the number of planets on which organisms could dwell, it is claimed.
On Earth, living microbes are found at depths of up to around five kilometres.
How far below the surface organisms can survive depends on warmth both from a planet's star, and interior geothermal heating.
Scientists have produced a computer simulation that takes these factors into account for icy and rocky worlds.
"We have developed a new model to show how Goldilocks zones can be calculated for underground water and hence life," said lead researcher Sean McMahon, from the University of Aberdeen's School of Geosciences.
"Many more habitable planets exist if you are prepared to take into account sub-surface life."
The research was presented today at the British Science Festival, taking place at the University of Aberdeen.
Mr McMahon, a PhD student, said even "rogue" planets thought to be floating in interstellar space, far from the warmth of any star, may be able to support sub-surface life.
If the planets were large enough their interiors could be warmed by geothermal forces alone.