Many of our galaxy's suns have destroyed the atmospheres of orbiting Earth-like planets—or so astrobiologists have long feared. The Milky Way, they note, is dominated by M dwarf stars: violent, unpredictable suns that frequently hurl high-energy particles and solar flares into space. Because they are much cooler than our sun, any potentially habitable planet would need to orbit them much closer than Earth does, putting it smack in the danger zone. But a new study indicates that these planets may be unexpectedly shielded from solar activity, keeping life safe.
"Overall, this is excellent news for planet hunters," says Alan Boss, a planetary scientist at the Carnegie Institution for Science in Washington, D.C., who was not involved with the study and who is part of NASA's Kepler mission to search for Earth-like planets. "This further buttresses the case that the first truly habitable world we find will likely by around a nearby M dwarf."
M dwarfs make up at least 70% of the Milky Way's stars. Their masses range from roughly half to one-twentieth the mass of our sun, but what M dwarfs lack in size, they more than make up for in longevity. Astronomers estimate that these stars can burn for 40 billion to 100 billion years, giving any habitable planets plenty of time to evolve life. (The life span of our own sun, a G-class star, is about 10 billion years.) But during at least the first few billion years of their lives, M dwarfs also sport huge magnetic fields that routinely interact with their atmospheres to create coronal mass ejections—enormous outbursts of matter from the star's highly ionized corona—and proton-rich flares.
To find out if this solar barbecue would sear any nearby habitable worlds, researchers led by astrobiologist Antigona Segura of the Universidad Nacional Autónoma de México (UNAM) in Mexico City turned to a computer model. The team simulated how a 1985 flare from AD Leonis (AD Leo), an M dwarf 16 light-years from Earth, would have affected a hypothetical Earth-like planet orbiting 0.16 astronomical units from the star. That's less than half Mercury's distance around the sun.
The simulation, which will be reported in an upcoming issue of Astrobiology, indicates that M dwarf stars are not as dangerous as feared. "When UV radiation from the star's upper atmosphere encountered the Earth-like atmosphere of our model planet, the energy resulted in a thicker ozone layer in the planetary atmosphere, providing a natural shield for the planetary surface," says astronomer Lucianne Walkowicz, one of the paper's co-authors and a Kepler postdoctoral fellow at the University of California, Berkeley. That's because the UV radiation actually split molecules of oxygen to create more ozone than it destroyed. "Throughout most of the flare, the surface of our model Earth-like planet experienced no more radiation than is typical on a sunny day here on Earth," Walkowicz says.
The findings are especially good news, says Segura, because AD Leo is a young star—less than 300 million years old—and as a result is one of the most active M dwarfs known. The star's 1985 flare was 1000 times as energetic as a similar flare on our own sun. So the fact that the model planet's atmosphere survived such a violent event may bode well for planets around similar young M dwarfs, she says.
Mark Giampapa, an astronomer at the National Optical Astronomy Observatory in Tucson, Arizona, praises the work, calling it "extremely interesting and timely." But he notes that most M dwarf stars are cooler than AD Leo is. Earth-like planets would have to orbit even closer to such stars and would thus suffer from stronger flare activity than the hypothetical planet in this study. In that case, he says, the planetary atmosphere may not go so unscathed.