Planet orbits a common dwarf star, suggesting more may be out there

By Nadia Drake

Web edition: August 31, 2012
Print edition: October 6, 2012; Vol.182 #7 (p. 12)

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Gliese 163c (illustration, left; false-color image, right) was recently observed orbiting a star 50 light-years away. The planet lies within its star's habitable zone.

Credit: PHL/UPR Arecibo, IRSA/IPAC/NASA

BEIJING — A potentially habitable planet has been discovered orbiting the star Gliese 163, 50 light-years away. The planet is bigger than Earth — roughly seven times as massive — and resides near the inner edge of the star’s habitable zone, Thierry Forveille of France’s Observatoire de Grenoble reported on August 30 at the International Astronomical Union’s general assembly meeting. Depending on its composition and how insulating its atmosphere is, the planet could be capable of supporting life.

“I’d say that’s a habitable planet,” said Raymond Pierrehumbert of the University of Chicago. It’s unlikely the planet would experience any sort of runaway greenhouse effect that would heat it beyond the point of livability, he says.

Forveille and his colleagues found the planet by searching for wobbles in the planet’s host star with a telescope in Chile. Astronomers calculate that Gl 163c, as the planet is called, receives 30 to 40 percent more energy than Earth receives from the sun. Because the planet’s radius is unknown, it’s not yet clear what the planet is made of, but scientists speculate that it’s a mix of rock and water.

Gliese 163 is an M dwarf star, is smaller and dimmer than the sun, and hosts at least two planets. The innermost planet, Gl 163b, is 11 Earth masses and completes a revolution in just 8.6 days; next out is Gl 163c, with 7 Earth masses and an orbital period of 25.6 days. And there’s a third potential planet, a 20-Earth-mass body much farther out, with an orbital period of 669 days.

Searching for habitable planets around M dwarfs could be the fast track to finding a real Earth analog, astronomers say. “It’s easier to find and follow up on an Earth-size planet in the habitable zone of an M dwarf,” said Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics.

These stars are smaller, so Earth-size planets are easier to find: They tug more on the star, and produce a bigger drop in brightness. And M dwarfs are common, comprising as much as 80 percent of the stars in the solar neighborhood. Because of that, there’s a good chance the Kepler spacecraft could spot an Earth-sized world around an M dwarf within 75 light-years from Earth, Dressing reported on August 30.

“This provides a lot of motivation for looking for Earth-sized planets around our smallest stars,” she said.