All the planets of the Solar System orbit the Sun in the same direction as it rotates around its axis. We thought it was the same for extrasolar planets around their stars, but six planets come to be caught spinning in the opposite direction to the rotation of their stars. The discovery of these so-called retrograde motion is in response to observations made largely by Amaury Triaud and Didier Queloz of the Geneva Observatory, and Andrew Cameron of the University of St Andrews (Scotland). It poses a serious problem for theories of planetary formation, which predict that planets in retrograde motion should not come into being.

Indeed, a planetary system is born from a cloud of gas and dust that spins as it contracts and flattens. At the center star is born, and the residual disk formed later planets. These must therefore rotate in the same direction as the star, and have orbits roughly in the same plane.

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