BioEd Online

The most common type of star in the Galaxy may be more hospitable to life than was previously believed, say astronomers who have calculated how much radiation planets orbiting such stars would receive.

Red dwarfs — cool, low-mass stars — make up more than 75% of the stars in the Milky Way. So far, ten planets have been found around red dwarfs, including one announced on 24 April that appears to be on the edge of a 'habitable zone', the region around a star thought to be capable of supporting life.

Many astronomers are sceptical that life could survive around red dwarfs. Because the stars are cooler than, say, our Sun, their habitable zones lie much closer to the star. That means that planets in the zone could be exposed to damaging levels of ultraviolet and X-ray radiation.

But life might be able to stand the radiation — if the planet was like Earth, say Edward Guinan of Villanova University, Pennsylvania, and his collaborators.

The researchers examined the X-ray and ultraviolet output of 20 red dwarfs and found that if a planet had an atmosphere to scatter ultraviolet radiation and a magnetic field to deflect harmful particles, any life would be spared most of the radiation. The results were released on 28 May at the American Astronomical Society's biannual meeting in Honolulu, Hawaii.

Hawaiian nights

Periodic solar flares could still deliver enough radiation to sterilize animals, says Guinan. Plus, 'tidal locking', which forces a moon or planet's rotation to synch with that of the body it orbits, would probably leave one side in continual, searing light.

But, Guinan says, winds could circulate warm air to the night side, keeping it as warm as a summer night in Hawaii.

And red dwarfs are very stable, so the planet could remain habitable for billions of years, unlike our Sun, which will support life only for another 1.5 billion years.

"It's not as bad a place as people thought," says Alan Boss, a theorist with the Carnegie Institution in Washington DC. Because red dwarfs are common throughout the Galaxy, Boss believes that future missions, such as NASA's Kepler satellite, scheduled for launch next year, may be able to spot the eclipse of Earth-sized planets around such a star.

But spotting life on such a planet would probably require even more powerful space-based telescopes.