Melbourne: All planets are accepted to orbit around their stars in the same direction as the star spins. But, now astronomers have discovered a heavenly body which they claim breaks the mould.

Led by Daniel Bayliss of Australian National University, the astronomers, using one of the world's largest telescopes in Chile have discovered that the distant planet, WASP-17b, is moving in opposite direction to the spin of the star around which it orbits.

WASP-17b is quite different, Dr Bayliss said, and its backwards motion is somewhat of a mystery to scientists.

"It is possible that the planet underwent a close encounter with another giant planet billions of years ago, which altered its orbit so much that it began orbiting backwards," he said.

The discovery throws traditional theories about how planets form around stars into doubt, say the astronomers.

Planets formed from the same disk of rotating material that gives birth to the star around which they move. So until now it has been assumed that any planets orbiting a star would be moving in the same direction as the star's spin. This is certainly true in our own Solar System.

It is not known what fraction of planets orbit their stars in this retrograde manner, but the astronomers are now actively trying to monitor other distant planets to see how common it is.

If it were common, this would not bode well for the chances of life around other stars. Close encounters between giant planets would most probably destroy any small Earth-like planet in that system and wipe out any chance of life arising.