Two U.S. citizen scientists have discovered a planet in a system with four different suns, the first known of its type, U.S. and British astronomers say.

The distant planet orbits one pair of stars while a second pair of stars orbits around it, they said.

The planet was discovered by two U.S. volunteers using the Planethunters.org website who spotted faint dips in light caused by the planet "transiting," or passing in front of, its parent stars.

U.S. and British astronomers then confirmed the discovery of the planet, thought to be a "gas giant" slightly larger than Neptune and more than six times the size of the Earth, with the Keck telescopes in Hawaii.

Astronomers say they're puzzled by how the planet avoids being pulled apart by the gravitational forces of its four stars.

"All four stars pulling on it creates a very complicated environment," Chris Lintott from the University of Oxford told BBC News. "Yet there it sits in an apparently stable orbit."

"That's really confusing, which is one of the things which makes this discovery so fun. It's absolutely not what we would have expected."