Subsurface Habitable Zones

The circumstellar habitable zone is a band of space in a solar system that is neither too far away nor too close to the sun for liquid water to exist on a planet's surface. The traditional habitable zone is relatively small; most planets in the universe will fall somewhere outside of their star's habitable zone. A factor that is not usually considered is that life can exist without liquid water on the surface level of a planet. It is possible for life to take it's energy from a planet's core rather than from the planet's star, and liquid water can more readily exist closer to that geothermal energy.

Sean McMahon, Jack O’Malley-James, and John Parnell have published a paper in which they calculate the range of a subsurface habitable zone (SSHZ) – a counterpart to the original habitable zone that becomes larger and larger the farther below a planet's surface you are willing to look for life. Life has been found on Earth living as far below our surface as 4 kilometers, and the SSHZ for an earth like planet at depths that extreme is over three times as wide as a habitable zone where you're only looking for life on a planet's surface.

McMahon, O'Malley-James and Parnell's calculations are exciting because based on this information it isn't unreasonable to believe that far more planets are capable of sustaining life in some form than we originally believed. One of the significant downsides to this however, is that the further below the planetary surface that you go, the harder it is for us to effectively test for the presence of life. So even though there is a much larger range in every solar system where we believe life could exist, actually verifying the existence of that life becomes more difficult accordingly.

Sources:
Sean McMahon, "Circumstellar Habitable Zones for Deep Terrestrial Biospheres"