New planet system uncovered 5,000 light-years away

Astronomers said they have discovered the first system in which two planets orbit two stars that in turn circle each other, creating a celestial traffic pattern that makes the length of day vary wildly on the terrestrial surfaces.

The solar system, called Kepler 47, has one star about the size of the Earth's sun and a second one two-thirds smaller, which orbit each other every 7.5 days, according to a report by NASA scientists posted online yesterday by the journal Science.

The two planets that revolve around the stars are bigger than Earth, and one may be far enough away from the suns to have water.

"The thing I find most exciting is the potential for habitability," said William Welsh, a professor of astronomy at San Diego State University. He presented the data at the International Astronomical Union meeting in Beijing.

Measurements showed variable daylight patterns as the two stars completed orbits that took them closer, then farther from the planets, which themselves travel in their own patterns around the two stars. The system is about 5,000 light-years away from Earth.

The outer planet in the double-star system, 4.6 times the size of Earth or about the size of Uranus, orbits every 303 days. That means it's in the so-called habitable zone, where liquid water could be present. The inner planet is three times bigger than Earth, orbiting every 49 days.

Neither planet is likely to harbor life; the inner one is too close, and the outer one is probably a gas giant. But if the outer planet has moons, "those would be very interesting worlds," Welsh said.

NASA's Kepler spacecraft was launched in March 2009 with its mission to discover Earth-size and smaller celestial bodies in regions around their stars where water might exist on the surface of the planet. The mission has confirmed at least 75 planets and identified 2,321 planet candidates.