NEW YORK — Astronomers said they discovered the first system where 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 Wednesday 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 in California. 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.