In The Field

AAS 2009: Don’t be shy, cuddle up to an M dwarf

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Some 70% of the stars in the galaxy are M dwarfs, and so exoplanet surveys will be targeting these relatively cool, small stars. But the habitable zone for Earth-like planets in these systems will be at least five times closer to the M dwarf than the Earth is to the sun. That keeps the planet warm, but also subjects it to the star’s capricious behavior. “They flare as powerfully or even more so than the Sun,” says Lucianne Walkowicz, of the University of California at Berkeley, who gauged the effects of an M dwarf flare on planetary habitability in an AAS poster session. “People assumed that this would just sterilize the planet.”

Lucianne modeled a 4-hour flare — a classic one that astronomers had observed carefully on a nearby M dwarf -and calculated its effects on an Earth-like planet, with an Earth-like atmosphere, orbiting at 0.16 astronomical units – six times closer than the Earth is to the Sun. She measured the effect of the flare — which for our Sun, can rise up to heights 50 times the diameter of the Earth — on temperature, ozone, UV flux and water vapour.

Turns out, the flare wasn’t that big of a deal. Temperatures only changed a tenth of a degree. Most of the energy of the flare went into the upper atmosphere where it broke down ozone. The fraction of the radiation that did make it through, she says, is less than you’d get on a sunny day walking around Earth.

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