Short Sharp Science - Triple-star system may host habitable world

Lisa Grossman, reporter

A potentially rocky planet (right) orbits a dwarf star (centre) that in turn orbits a pair of more distant suns (left) in this illustration (Image: Guillem Anglada-Escudé)

Astronomers have found the first potentially habitable planet in a triple-star system.

The planet, unromantically named GJ 667Cc, orbits a small, dim dwarf star 22 light years away. That star in turn orbits a pair of sun-like stars that lie about as far away from it as Pluto lies from our sun. The stellar pair would shine more brightly than any others in the planet's night sky.

More than 100 planets have been found in their stars' habitable zones, where water can remain liquid. But only a handful of these are strong candidates for being rocky like Earth rather than gassy like Jupiter, making them better candidates for hosting life.

The other rocky candidates lie at the very edges of their stars' habitable zones. But GJ 667Cc is just right. "It lies right in the middle of this habitable zone, roughly where Earth would be in that solar system," says Guillem Anglada-Escudé of the University of Gottingen in Germany.

The planet is at least 4.5 times as massive as Earth and orbits its host star once every 28.15 days. It revealed itself by gravitationally tugging its host star back and forth, creating observable wobbles in the star's spectrum.

Unfortunately, this technique only puts a lower limit on the planet's mass – it could be up to 9 times as massive as Earth, Anglada-Escudé says. "It might not have a rocky surface," he admits.

To nail down its composition, astronomers need both its mass and its physical size – which cannot be measured in this case because of the tilt of the planet's orbit. "Only with those two pieces of information can the density be determined, to show the planet as solid rock," says Geoff Marcy of the University of California Berkeley, who was not involved in the new discovery.

But the fact that the planet is there at all suggests that low-mass planets are quite easy to build. Despite the fact that the star is small and doesn't contain a lot of heavy elements, which are the building blocks of planets, it apparently hosts two other super-Earth-sized planets outside of the habitable zone, with orbital periods of 7.2 and 75 days. There may also be a more distant gas giant planet. "This probably means that these planets can form easily around any kind of star," Anglada-Escudé says. 

http://www.newscientist.com/