The Habitable Zone for Exoplanets May Be 14 Times Wider Than We Thought

Artistic depiction of Kepler 62f via NASA Ames/JPL-Caltech.

It's hard to believe that the Kepler space observatory has only been in orbit for five years. The plucky spacecraft has discovered hundreds of exoplanet candidates in that short time, even as it's dealt with trying technical challenges. It's also engaged space enthusiasts across the globe with its ever-growing list of discoveries, and has sparked vivacious discussion among experts about the nature of habitable planets in other solar systems.

Kepler has pinpointed many planets in the “Goldilocks zone,” meaning those worlds are close enough to their star to support liquid water, but not so close that the water evaporates into space. Earth is our only dataset when it comes to the development of life, so it makes sense to prioritize these Goldilocks planets as the most likely potential biospheres.

But many scientists want to buck the trend and pan for extraterrestrial life in weirder places. Such is the case with a research team based at the University of Aberdeen (with some help from St. Andrews University). The team published a study in Planetary and Space Science suggesting that subsurface biospheres can survive far beyond the Goldilocks zone.

Various orbits of Kepler planetary systems via NASA.

“Life on Earth is not restricted to the surface and includes a 'deep biosphere' reaching several kilometers in depth,” the team explained in the paper's abstract. “Similarly, subsurface liquid water maintained by internal planetary heat could potentially support life well outside conventional [habitable zones].”

The team introduced a new term—“subsurface-habitability zone”—to describe the expanded range of orbital distances that could support life in deep, subterranean environments. To test the idea, they created a computer model that approximates underground temperatures of planets of a given size and distance from the sun.

“The deepest known life on Earth is 5.3 kilometers below the surface, but there may well be life even ten kilometers deep in places on Earth that haven’t yet been drilled,” said PhD student Sean McMahon to Phys.org.

“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 kilometers below the planet surface … If we go deeper, and consider the top ten km below the Earth’s surface, then the habitable zone for an Earth-like planet is 14 times wider.” In other words, exoplanets that have been written off as barren wastelands may host life deep beneath inhospitable surfaces.

Applying this idea to our own solar system is just as much fun as considering it for exoplanets. The Goldilocks zone extends to Mars, but tripling the parameters would put the tantalizing Jovian moons into the subsurface-habitability zone. Increasing it 14-fold would throw the moons of Saturn and Uranus into the mix too.

The tardigrade, a practically indestructible extremophile. Photo via Bob Goldstein and Vicky Madden, UNC Chapel Hill

This study's conclusions should come as no surprise to astrobiologists specializing in extremophiles. In the last year alone, organisms were found hanging out in subglacial lakes in Antarctica as well as sterilized spacecraft clean rooms.

Sure, these creatures are not building radio telescopes or launching themselves to the moon. But they do demonstrate that life, uh, finds a way. So let's stop with the surface elitism, and consider that life may be flourishing where the exo-sun don't shine.