Smivs writes "BBC News is reporting that astronomers have discovered the first planet that orbits in the opposite direction to the spin of its star. Planets form out of the same swirling gas cloud that creates a star, so they are expected to orbit in the same direction that the star rotates. The new planet is thought to have been flung into its "retrograde" orbit by a close encounter with either another planet or with a passing star. The work has been submitted to the Astrophysical Journal for publication. Co-author Coel Hellier, from Keele University in Staffordshire, UK, said planets with retrograde orbits were thought to be rare. '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.' Professor Hellier said a near-collision was probably responsible for this planet's unusual orbit. 'If you have a near-collision, then you'll have a large gravitational slingshot from that interaction,' he explained. 'This is the likeliest explanation. But it might be possible you can do it by gradually perturbing the orbit through the influence of a second planet. So far, we haven't found any evidence of a second planet there.'"

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