The consensus among scientists is that for a planet to be capable of harbouring life, it must have surface water and reside in the ”Goldilocks zone” – where it’s not too hot and not too cold.
But researchers from the University of Aberdeen suggest that many more planets than previously thought may contain life, because it could be thriving beneath the surface and kept warm by a hot molten core.
Just as it is on Earth.
Professor John Parnell, from Aberdeen University, told an audience at the city’s British Science Festival: "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."
Professor Parnell, along with PhD student Sean McMahon, is developing a model that star gazers could use to pinpoint planets capable of harbouring sub-surface life, the BBC reported.
These would be bodies too close to a star for surface water to exist, or too far away for it to remain in liquid form.
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."
Another boost in the search for alien life came earlier this year, when an international team of researchers, led by Dr Xavier Bonfils, from Grenoble University in France, suggested that there could be billions of habitable planets lurking in the Milky Way.
They found nine “Super Earths” in orbit around red dwarf stars, with two of these in the Goldilocks zone.
They reckon that red dwarfs make up 80 per cent of the 200 to 400 billion stars in our galaxy and that 40 per cent of these could be gently warming habitable planets.
So far scientists have confirmed over 700 exo-planets - bodies outside our solar system - with another 1,000 unconfirmed objects.
Nasa's Kepler mission, launched in 2009, is at the forefront of the search for habitable planets.