Cold, remote Earth-size planets may harbour life

Remote Earth-size planets, believed to be uninhabitable, may be able to support life beneath a cold, barren and rocky surface, according to a new research.
The study, by researchers at the universities of Aberdeen and St Andrews in UK, challenges the traditional habitable zone by suggesting that it is possible for life to exist deep below the ground.
Sean McMahon, at the University of Aberdeen, said the current understanding of the typical climate planets require, which is a balance between not being too close to its sun and not too far away, does not "take into account" life that may 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," he said.
McMahon explained the surfaces of rocky planets and moons differ to Earth. Such planets are typically cold and barren with no atmosphere, or have a very thin or even corrosive atmosphere.
Yet, living below the surface protects organisms from unpleasant conditions, allowing the subsurface habitable zone to be potentially very important, 'The Times' reported.

"Earth might even be unusual in having life on the surface," he said.
The team created a computer model to estimate the temperature below the surface of a planet of a given size and at a given distance from its star.

The deepest known life on Earth exists at 5.3 km below the surface but the research shows there is a possibility life exists as much as 10 km deep on parts of the planet which have yet to be 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," McMahon said.
"The model shows that liquid water and as such life could survive 5 km below the Earth's surface, even if the Earth was three times further away from the sun than it is just now," he said.

The current habitable zone for the solar system extends out to Mars, yet this re-drawn habitable zone would see the zone extend out further than Jupiter and Saturn.
The study was published in Planetary and Space Science.