W. G. Henning/NASA

“Finding multiple planets in the habitable zone of their host star is a great discovery because it means that there can be even more potentially habitable planets per star than we thought. Finding more rocky planets in the habitable zone—per star—increases our odds of finding life.” Cornell’s Lisa Kaltenegger is referring to last week’s news of TRAPPIST-1, a planetary system with seven Earth-size exoplanets—three in the habitable zone—only 40 light years away. Kaltenegger and her colleague, Ramses Ramirez, may be adjusting that habitable zone slightly.

The two report this week in the Astrophysical Journal Letters that hydrogen pouring from volcanic sources on planets could improve the chances of locating life in the cosmos.

Combining the greenhouse warming effect from hydrogen, water, and carbon dioxide on planets sprinkled throughout the cosmos, distant stars could expand their habitable zones by 30 to 60 percent, according to their research. “Where we thought you would only find icy wastelands, planets can be nice and warm—as long as volcanoes are in view,” says Kaltenegger.

Planets located great distances from stars freeze over. “On frozen planets, any potential life would be buried under layers of ice, which would make it really hard to spot with telescopes,” says lead author Ramirez. “But if the surface is warm enough—thanks to volcanic hydrogen and atmospheric warming—you could have life on the surface, generating a slew of detectable signatures.”

The idea that hydrogen can warm a planet is not new, but an Earth-like planet cannot hold onto its hydrogen for more than a few million years. Volcanoes change the concept. “You get a nice big warming effect from volcanic hydrogen, which is sustainable as long as the volcanoes are intense enough,” explains Ramirez, who suggests the possibility that these planets may sustain detectable life on their surface.

A very light gas, hydrogen also “puffs up” planetary atmospheres, which will likely help scientists detect signs of life. “Adding hydrogen to the air of an exoplanet is a good thing if you’re an astronomer trying to observe potential life from a telescope or a space mission. It increases your signal, making it easier to spot the makeup of the atmosphere as compared to planets without hydrogen,” Ramirez continues.

In our solar system, the habitable zone extends to 1.67 times the Earth-Sun distance, just beyond the orbit of Mars. With volcanically sourced hydrogen on planets, this could extend our solar system’s habitable zone reach to 2.4 times the Earth-Sun distance—about where the asteroid belt is located between Mars and Jupiter. This research places a lot of planets that scientists previously thought to be too cold to support detectable life back into play.

“We just increased the width of the habitable zone by about half, adding a lot more planets to our ‘search here’ target list,” concludes Ramirez.

Atmospheric biosignatures, such as methane in combination with ozone—indicating life—will likely be detected by the forthcoming, next-generation James Webb Space Telescope, launching in 2018, or the approaching European Extremely Large Telescope, which will achieve first light in 2024.

Image: W. G. Henning/NASA

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