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Extra-terrestrial life on alien worlds could be far more widespread than previously thought, researchers have said.
According to a study by experts at the University of Aberdeen, the number of habitable Earth-like planets in the universe could be far greater than is currently thought.
Published in Planetary and Space Science, the team challenge traditional ideas about "habitable zones" – the area of space around a star or sun that can support life – by looking at life below ground.
PhD student Sean McMahon explained: "A planet needs to be not too close to its sun but also not too far away for liquid water to persist, rather than boiling or freezing, on the surface.
"But 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."
Using a computer model that includes the top five kilometres below the surface of the planet, researchers found the habitable zone for an Earth-like planet is three times bigger than currently believed.
If looking at the top 10km of a planet's surface, the habitable zone in our solar system would extend out further than Jupiter and Saturn – 14 times more than current thinking.
McMahon also noted that life could be present on "rogue" planets that drift around in darkness: "Rocky planets a few times larger than the Earth could support liquid water at about five km below the surface even in interstellar space (i.e. very far away from a star), even if they have no atmosphere because the larger the planet, the more heat they generate internally.
"It has been suggested that the planet Gliese 581 d, which is 20 light years away from Earth in the constellation Libra, may be too cold for liquid water at the surface. However, our model suggests that it is very likely to be able to support liquid water less than 2 km below the surface, assuming it is Earth-like."
McMahon said he hopes more researchers will use the study to consider how life might be found on other planets: "The results suggest life may occur much more commonly deep within planets and moons than on their surfaces. This means it might be worth looking for signs of life outside conventional habitable zones.
"The surfaces of rocky planets and moons that we know of are nothing like Earth. They're typically cold and barren with no atmosphere or a very thin or even corrosive atmosphere. Going below the surface protects you from a whole host of unpleasant conditions on the surface. So the subsurface habitable zone may turn out to be very important. Earth might even be unusual in having life on the surface."
Speaking to IBTimes UK, McMahon said that while we currently have no evidence of life outside our planet, it is feasible: "We have a convention of saying that where you have liquid water and the right sort of temperatures and possible sources of food and energy – in those sorts of conditions life could survive.
"What we've found is that those types of conditions are probably very common below the surface and maybe more so than they are on the surfaces. But that means that life can originate in these environments below the surface of rocky planets."
Explaining why the idea of life below the surface has not been considered outside our solar system before, he said: "It's partly a historical thing that when the idea of a habitable zone was first formulated we did not know as much about life below the surface.
"We're used to thinking about the possibility of life below the surface in our own solar system, but we've stuck with this notion of a habitable zone which was formulated in the early 90s based around the idea that life is something that happens on the surface. Since then we've discovered that life goes much further below the surface of the Earth.
"In our own solar system the best candidates for inhabited planets and moons are Mars and some of the moons of Jupiter and Saturn, which have ice on the outside and liquid water below. If there is life on Mars today it's probably quite far below the surface, where it's still warm."
And considering what life we might find festering underneath the surface of alien worlds, he added: "It depends on how much energy is available. On some planets and moons there might be quite a lot but on others there would not be much. The other problem is that there is not much space in the cracks and pores of rocks for larger life forms to grow and become complex.
"If there is not much, you wouldn't find anything more than bacteria, fungus and perhaps worms – the sorts of things you find several miles below the earth's surface."