The nearby star system Fomalhaut, with its exoplanet and dusty debris disk, is not just a double star, it's an unusual triple star, a U.S astronomer reports.

Eric Mamajek, a professor of physics and astronomy at the University of Rochester, and his collaborators found a star significantly distant from the Fomalhaut pair is in fact part of a three star system.

"I noticed this third star a couple of years ago when I was plotting the motions of stars in the vicinity of Fomalhaut for another study," Mamajek said in a university release Thursday. "However I needed to collect more data and gather a team of co-authors with different observations to test whether the star's properties are consistent with being a third member of the Fomalhaut system."

Careful analysis of the star's precise movements and spectroscopic measurements of its temperature and radial velocity allowed the researchers to measure the distance and speed of the third star.

They concluded the star, until recently known as LP 876-10, is part of the Fomalhaut system, making it officially Fomalhaut C.

"Fomalhaut C looks quite far apart from the big, bright star that is Fomalhaut A when you look up at the sky from Earth," Mamajek said.

"Fomalhaut A is such a massive star, about twice the mass of our Sun, that it can exert sufficient gravitational pull to keep this tiny star bound to it -- despite the star being 158,000 times farther away from Fomalhaut than the Earth is from the Sun," he said.

Despite being a well-studied system, the researchers said, it was only recently confirmed that Fomalhaut was a binary star, a suggestion first made in the 1890s.

And now it's officially a triple.