Astronomers scanning the skies for another Earth might need to narrow their search. New research suggests that even if a world lies within the Habitable Zone, in which water is liquid, too much or too little volcanic activity can render it lifeless.
When assessing a distant planet's habitability, astronomers currently focus on one main criterion: Could the planet have liquid water on its surface? Too close to its sun, and that water evaporates away; too far, and it's locked in ice.
But the equation isn't quite that simple, says planetary scientist Rory Barnes of the University of Washington, Seattle. In an upcoming issue of The Astrophysical Journal Letters, he and colleagues argue that E.T. seekers also need to throw volcanoes into the mix.
We wouldn't be here without volcanoes. Early in Earth's history, volcanic eruptions spewed carbon dioxide and water vapor from deep beneath the surface, creating conditions that would eventually support photosynthesis. Mars, with an inactive core and hence little volcanism, wasn't so lucky. But too much volcanic activity can also be bad: Jupiter's moon Io is jostled so much by the gravitational pull, or tidal force, of its gigantic parent and neighboring moons that its volcanoes erupt almost continuously--enough to coat the moon's surface with fresh lava about every million years. These eruptions presumably would snuff out any incipient life.
Barnes and colleagues applied this thinking to an extrasolar planet called GJ 581 d, located about 20 light-years away and discovered in 2007. All traditional signs point to it being potentially habitable: It's a rocky planet, and it's just far enough away from its star to sport liquid water (at least theoretically). But the researchers found a tidal-forces problem: Their study of GJ 581 d's orbital data suggest some tidal pulling is at work, possibly from planetary neighbors as well as its sun, but not enough to spark sufficient volcanism. So the tidal forces aren't strong enough to permit habitability, Barnes says.
The surprising result of the research, says co-author Brian Jackson, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, "is that many planets that are in the traditional Habitable Zone ... might not be habitable at all." Equally as fascinating, Barnes says, is "that, without any data regarding [the planet's] composition, we are starting to understand the interiors of exoplanets."
For good or ill, there's no doubt tidal forces play an important part in the biological potential of planets, says astronomer Darren Williams of the Pennsylvania State University, Erie. "The litmus test for planetary habitability has until now been adequate sunlight and amenable surface climate," he says. But this paper expands the definition to include the warming effects of tidal forces.