retrograde orbit exoplanetIn the search for planets beyond our solar system, the UK's Wide Area Search for Planets (WASP) has stumbled upon a bizarre-o world that is orbiting its star in reverse. 

"I have to say this is one of the strangest planets we know about," Sara Seager, an astrophysicist from the Massachusetts Institute of Technology, told SPACE.com.

After stars spin up, they usually bring in nearby debris, which picks up the same directional orbit. "With everything [in the star system] swirling around the same way and the star spinning the same way, you have to do quite a lot to it to make it go in the opposite direction," Coel Hellier, of the UK's Keele University, told the BBC.

Indeed, the newly discovered exoplanet likely took a large close call with another even larger object to swing the planet (named WASP-17b) into a retrograde orbit. "If you have a near-collision, then you'll have a large gravitational slingshot from that interaction," Hellier said.

It's the first known planet to have such an unexpected orbit although some of the other planets' moons in our solar system fly a backward track around their planets.

Astronomers discovered WASP-17b's retrograde orbit by watching the star it orbits. "If you look at how the spectrum of the star changes when the planet transits across it," Hellier said, "you can work out which way the planet is traveling."

He and a team of researchers also calculated the gaseous planet's size (by checking the extent of the star's movement as the planet passed) as twice as big as Jupiter but with a mass of about half of that planet.  The low density could be explained by the near miss with another large object, or by the planet's long elliptical orbit, which brings it close to its weighty star.

"I think it's extremely exciting," Seager, who wasn't involved in the find, told SPACE.com. "It's fascinating that we can study orbits of planets so far away." This gaseous giant is about 1,000 light years away. Seager was glad to see proof of this phenomenon. "There's always theory, but there's nothing like an observation to really prove it," she said.

Artistic rendering of another exoplanet orbiting close to its star courtesy of ESA/NASA/M. Kornmerrer/STScl