New research by NASA scientists suggests the so-called habitable zone surrounding red dwarf stars may not be as habitable as previously thought.
The habitable zone is the area surrounding a star in which liquid water can exist on an orbiting exoplanet, thus presenting the possibility for life. The zone’s dimensions are defined by the size and strength of the star. An exoplanet in the habitable zone is far enough from its sun that water isn’t boiled away, but close enough that it isn’t perpetually frozen.
Astronomers still study exoplanets outside of habitable zones in order to better understand our own solar system. But for most exoplanet-hunters, the goal is to find alien worlds that might sustain life.
“If we want to find an exoplanet that can develop and sustain life, we must figure out which stars make the best parents,” Vladimir Airapetian, a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., said in a news release. “We’re coming closer to understanding what kind of parent stars we need.”
Exoplanet surveys often concentrate on red dwarfs because they’re generally smaller, cooler stars with more intimate habitable zones, making their exoplanets easier to spot. But new research suggests current habitability models underestimate the risks faced by exoplanets orbiting so closely to red dwarfs.
Red dwarfs host more stellar activity than our own star. Solar flares and solar storms can ionize gases in the upper atmosphere, allowing elements to escape. Most models suggest only the lightest elements, like hydrogen, would escape in significant amounts, leaving behind larger concentrations of heavier elements essential to life, like oxygen and nitrogen. But heavier elements aren’t immune to ion escape.
“We know oxygen ion escape happens on Earth at a smaller scale since the sun exhibits only a fraction of the activity of younger stars,” said Alex Glocer, a Goddard astrophysicist. “To see how this effect scales when you get more high-energy input like you’d see from young stars, we developed a model.”
New models suggest astronomers have underestimated the volatility of young red dwarfs. Researchers showed the barrage of solar radiation from a red dwarf could slowly rob a habitable exoplanet of its hydrogen and oxygen, depleting its water supply.
Researchers published their findings in the Astrophysical Journal Letters.
“We have pessimistic results for planets around young red dwarfs in this study, but we also have a better understanding of which stars have good prospects for habitability,” Airapetian said. “As we learn more about what we need from a host star, it seems more and more that our sun is just one of those perfect parent stars, to have supported life on Earth.”
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