When you think of a volcano erupting, your first thought is probably more "run for your life" than "good for life." But according to a new study by Cornell University astronomers, planets with heavy volcanic activity might offer bastions for life throughout the cosmos, even if those worlds orbit outside the habitable zones of their stars.
Liquid water is supposed to be the key to life, not fire. That's why astronomers tell us that life is most likely to form on planets that are close enough to their stars to melt water, but not so close as to burn it all off. This narrow strip is known as the "habitable zone," where liquid water — and thus life — can thrive.
Distance from a star isn't the only factor that regulates a planet's temperature, though. Volcanic activity can also play a role. When volcanoes erupt, they spew out loads of greenhouse gases such as hydrogen and carbon dioxide that can warm a planet's surface, even beyond the habitable zone. Global warming, if you live too far away from your star, can be a good thing.
"Where we thought you would only find icy wastelands, planets can be nice and warm — as long as volcanoes are in view,” said Lisa Kaltenegger, Cornell professor of astronomy and director of the Carl Sagan Institute.
Volcanic hydrogen is the key
Here on Earth, the crucial greenhouse gas worth monitoring is carbon dioxide. For exoplanets that orbit outside their habitable zones, however, the key gas would be volcanic hydrogen.
“You get a nice big warming effect from volcanic hydrogen, which is sustainable as long as the volcanoes are intense enough,” explained Ramses Ramirez, one of the study authors.
The reason volcanic hydrogen is so important (as opposed to just regular old hydrogen) is that hydrogen is an extremely light gas. It's so light, in fact, that for any Earth-sized planet, it floats off into space because gravity isn't strong enough to retain it. But if a planet hosted volcanoes that were constantly erupting, spewing new hydrogen into the atmosphere, then the hydrogen that gets lost can be replaced.
So if life does exist outside the habitable zones of other solar systems, then planets with heavy volcanic activity would be the places to look. In our solar system, the habitable zone extends just beyond the orbit of Mars. If there was a planet out there with volcanically sourced hydrogen, however, it could extend the habitable zone out to somewhere between Mars and Jupiter — basically where the asteroid belt exists.
“We just increased the width of the habitable zone by about half, adding a lot more planets to our ‘search here’ target list,” said Ramirez.
“Finding more rocky planets in the habitable zone — per star — increases our odds of finding life,” added Kaltenegger.