Most common stars are more life-friendly than thought

SMALL and cool they may be, but red dwarfs, the most common kind of stars, are more likely to support life than we thought. Far-off icy planets that orbit these stars could still be warm enough to contain liquid water because of the way snow and ice absorb their near-infrared light.

Alien-hunting astronomers tend to look for planets in the "habitable zone", the range of distances from a star where temperatures are balmy enough for water to be in liquid form but not so hot that it boils off.

Red dwarfs are cooler than stars like our sun, so their habitable zones were thought to be smaller. This should make any planet's gravitational tugs on the star stronger and habitable worlds easier to spot - if there are any. Red dwarfs tend to flare up, scorching any nearby planets. Those far enough away to be safe, meanwhile, could be too cold.

Now, climate scientist Manoj Joshi of the University of Reading, UK, and Robert Haberle of NASA's Ames Research Centre in California have discovered how red dwarfs could warm far-off planets.

Ice and snow reflect 50 to 80 per cent of visible light so the ice on Earth bounces most sunlight back to space, and stays frozen. Compared with our sun, red dwarfs shine less in visible wavelengths, and brighter in the near-infrared, which ice and snow soak up. "If there's ice or snow on the ground, more of the radiation that hits the surface will be absorbed," Joshi says. "Which means that you'd expect it to be a lot hotter than it would be otherwise."

Joshi and Haberle took spectral data for two red dwarfs known to have exoplanets, Gliese 436 and GJ 1214. They found that an icy planet would reflect 10 to 40 per cent of the light it receives from that star. Effectively, snow on those worlds is half as shiny, and the outer edge of their habitable zone is 10 to 30 per cent further out from those stars than thought. "You've got more area, more chance of getting a planet in the habitable zone," Joshi says. The work will be published in Astrobiology.

Does the result bring any known exoplanets in from "too cold" to "just right"? Not just yet. Although exoplanet Gliese 581d is a possibility, Sara Seager at the Massachusetts Institute of Technology points out that this planet is not thought to have much ice.

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