A team of researchers at the University of Aberdeen have calculated that planets once thought to be uninhabitable could in fact harbour living organisms deep within the ground.
The group used a computer model to predict what a world would have to be like to support life deep within its crust.
And their experiment has raised the bar on limits of the traditional life-supporting 'habitable zone' for planets orbiting around distant stars. Our own solar system shows that if a planet orbits too close to a star, its atmosphere becomes superheated, like Mercury, or frigid, like Mars.
But team member PhD student Sean McMahon said: "That theory fails to take into account life that can exist beneath a planet's surface. As you get deeper below a planet's surface, the temperature increases, and once you get down to a temperature where liquid water can exist - life can exist there too."
The researchers created a computer model that estimates the temperature below the surface of a planet of a given size, at a given distance from its star.
Mr McMahon added: "The deepest known life on Earth is 5.3km below the surface, but there may well be life even 10km deep in places on Earth that haven't yet been drilled. Using our computer model, we discovered that the habitable zone for an Earth-like planet orbiting a sun-like star is about three times bigger if we include the top five kilometres below the planet surface.
"The model shows that liquid water, and as such life, could survive 5km below the Earth's surface even if the Earth was three times further away from the sun than it is just now."
He added that the planet Gliese 581 d, which is 20 light years away from Earth and is thought to be dead, could now be considered a candidate for life as it could harbour liquid more than 2km below the surface.