When it comes to prime real estate for liquid water in the cosmos, location is everything. Too close to a star, and a planet’s water evaporates from the extreme heat. Too far, and the planet turns into an ice ball. The middle is the place to be. Anything beyond that outermost edge, where planets can’t retain enough warmth, is icy and desolate.
Or maybe not. In a study published Monday in the Astrophysical Journal Letters, a pair of researchers suggest a geological phenomenon could keep some planets beyond that edge warm enough to support liquid water—and maybe even life. They argue that the habitable zone, the lucky strip of space where life could emerge, may be bigger than previously thought.
The geological phenomenon is a well-documented one here on Earth: volcanoes. The researchers suggest that volcanic activity on a given exoplanet, spurred by the shifting of tectonic plates, could churn out enough hydrogen to warm its atmosphere even further out in a star system, with only little help of the parent star’s heat. Hydrogen is a very light gas, so the planet would be constantly losing it to space. But volcanoes could spew out hydrogen at a rate that keeps up with that loss, creating a steady amount of hydrogen in the atmosphere capable of helping to heat the planet. That warmth, in turn, could create conditions favorable for liquid water and other chemical compounds on the planet’s surface that would spark life. Using climate models, the researchers found the distance from a star at which volcanoes create enough heat through this process without freezing over. That spot constitutes a new outer edge of the habitable zone, about 50 percent wider than before.
“It really increases the number of potentially habitable planets there could be,” says Ramses Ramirez, one of the study authors and a research associate at Cornell University’s Carl Sagan Institute.
The presence of hydrogen “puffs out” atmospheres, which makes other chemical signatures easier to detect using powerful technology, like next-generation infrared telescopes. Ramirez says previous research has suggested hydrogen as a potential source of heat for planets far from their parent stars, but it stopped short of describing the mechanism by which the planet would hold onto that hydrogen.
For the purposes of our solar system, this research doesn’t change anything. Accounting for volcanic hydrogen pushes the outer edge of the system’s habitable zone to somewhere between Mars and Jupiter, the home of asteroids too small and cool for any hidden geological processes. The line sits not quite halfway to Jupiter, which means the gas giant’s moons don’t make the cut.
The research does carry some implications for another star system about 40 light-years away from ours: TRAPPIST-1, the system of seven Earth-sized planets that had the internet in awe last week. The astronomers studying TRAPPIST-1 predict at lease three of the seven planets orbit in the system’s habitable zone, but the presence of volcanic hydrogen there could make it four.
Lisa Kaltenegger, the study co-author and the Cornell professor who runs the Carl Sagan Institute, says the TRAPPIST-1 authors were thrilled when she presented them with this research.
“The more [habitable planets] we have, the easier—the better—our chances for finding life out there,” she says.
