The planet, located about 5,000 light years from Earth, has been dubbed PH1 in honor of Planet Hunters, a programme led by Yale University in the United States, which enlists volunteers to look for signs of new planets.

PH1 is orbiting two suns, and in turn is orbited by a second distant pair of stars. Only six planets are known to orbit two stars, researchers say, and none of those are orbited by other distant stars.

"Circumbinary planets are the extremes of planet formation," said Yale's Meg Schwamb, lead author of a paper presented Monday at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Nevada.

"The discovery of these systems is forcing us to go back to the drawing board to understand how such planets can assemble and evolve in these dynamically challenging environments."

US citizen scientists and Planet Hunters participants Kian Jek and Robert Gagliano were the first to identify PH1. Their observations were then confirmed by a team of US and British researchers working in Hawaii.

PH1 is a gas giant with a radius about 6.2 times that of Earth, making it slightly larger than Neptune. It orbits a pair of eclipsing stars that are 1.5 and 0.41 times the mass of the Sun roughly every 138 days.

The two other stars are orbiting the planetary system at a distance that is roughly 1,000 times the distance between Earth and the Sun.

The Planethunters.org website was created in 2010 to encourage amateur astronomers to identify planets outside our solar system, using data from the US space agency NASA's Kepler space telescope.

Kepler, launched in March 2009, is NASA's first mission in search of Earth-like planets orbiting stars similar to our Sun.

The discovery of PH1 was made available online Monday at the site arxiv.org and has been submitted to the Astrophysical Journal for publication.

"It still continues to astonish me how we can detect, let alone glean so much information, about another planet thousands of light-years away just by studying the light from its parent star," Jek said.

Last week scientists reported the discovery of a "diamond planet" twice the size of earth and orbiting a sun-like star.

Up to one-third of the planet's mass and much of its surface is believed to consist of diamonds, implying that distant rocky planets can no longer be assumed to have the same features as Earth.