A team of astronomers led by a U.S. graduate student has discovered a planet that shouldn't be where it is, raising questions about how planetary systems form.

The giant planet orbiting its star at 650 times the average Earth-sun distance if the most distantly orbiting planet found to date around a single, sun-like star, a University of Arizona release said Thursday.

More than 10 times the mass of Jupiter, the planet HD 106906 b is unlike anything in our own Solar System and throws a wrench in planet formation theories, the astronomers said.

"This system is especially fascinating because no model of either planet or star formation fully explains what we see," said researcher leader and UA graduate student Vanessa Bailey.

Several hypotheses have been put forward to explain the unexpected finding, including a mini binary star system.

"A binary star system can be formed when two adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitation attraction and bind them together in an orbit," Bailey explained. "It is possible that in the case of the HD 106906 system the star and planet collapsed independently from clumps of gas, but for some reason the planet's progenitor clump was starved for material and never grew large enough to ignite and become a star."

This system is also intriguing astronomers because they can still detect the remnant "debris disk" of material left over from planet and star formation, she said.

"Systems like this one, where we have additional information about the environment in which the planet resides, have the potential to help us disentangle the various formation models," Bailey said. "Future observations of the planet's orbital motion and the primary star's debris disk may help answer that question."