Hospitable planets may be more common than we thought, according to a team of researchers at the University of Aberdeen and University of St. Andrews. Their recent paper explains that most efforts looking for alien life have focused on the surface of planets, but the life on Earth extends down at least 5 kilometers and could extend down as far as ten, making that limitation unnecessary, the BBC reports.

Underground Life Could Extend Habitable Zone To Further Away Planets

Life on a planet’s surface could be the exception

Most astronomers have been working with the assumption that a planet must lie in a belt surrounding the nearest star called the ‘Habitable Zone’ in order to support life because if it’s any closer, water on the surface will boil away, and if it’s any further water on the surface will freeze.

But planets get warmer as you descend into them, so a planet could lie outside this belt and still have liquid water under the surface. Even though we live on the surface, there’s really no reason to expect other forms of life to do the same. We could in fact be the exception to the rule. Taking the top 10 kilometers of a planet’s surface into account, instead of just the very top, makes the Habitable Zone fourteen times larger and increases the number of planets that could potentially support life immensely (n.b. life doesn’t necessarily mean little green men; alien bacteria would be just as exciting for scientists, if less interesting for Hollywood).

Looking for life under the surface

Doctoral student Sean McMahon hopes to influence other astronomers, but to avoid the ambiguity and semantic debates that sometimes trip up academics, he has introduced the term ‘sub-surface habitability zone’ to make a clear distinction from the habitable zone that is already an accepted concept.

“Life may occur much more commonly deep within planets and moons than on their surfaces,” McMahon told the BBC. “It might be worth looking for signs of life outside conventional habitable zones…I hope people will study the ways in which life below the surface might reveal itself.”

Since astronomers are necessarily limited by what they can see through a telescope, it certainly isn’t obvious how to discern which planets really are just floating rocks and which have something more interesting going on under the surface, but knowing that the question exists can at least get the ball rolling.