The seventeenth planet discovered by the SuperWASP team is not only the largest discovered to date, but it also orbits its host star the wrong way, casting new light on how planetary systems form and evolve.

The planet, known as WASP-17, was discovered using the SuperWASP (Wide Area Search for Planets) South camera hosted by the South African Astronomical Observatory. Scientists at the Geneva Observatory then made follow up observations to confirm its status as a planet, and noticed its curious trait, its backward – or retrograde – orbit around its host star.

Since planets form out of the same swirling mass that creates a star, they are expected to orbit in the same direction as the star spins. Scientists speculate that the planet's orbit may have been altered when a nearby passing object spun it around in a giant game of planetary billiards.

Professor Coel Hellier of Keele University, remarks: "Shakespeare said that two planets could no more occupy the same orbit than two kings could rule England; WASP-17 shows that he was right."

Newly formed solar systems can be violent places, and a collision between a Mars-sized body with the Earth is thought to have generated enough debris to spawn our own Moon. "A near collision during the early, violent stage of this planetary system could well have caused a gravitational slingshot, flinging WASP-17 into its backwards orbit," says David Anderson of Keele University

WASP-17's other striking characteristic is its dominating size. At twice the size of Jupiter it is surprisingly light, with the density equivalent to that of polystyrene. Why some some extrasolar planets are far bigger than expected has been a bit of mystery, but WASP-17 points to a possible explanation. Forced into a highly elliptical, retrograde orbit, it would have been subjected to intense tides that would have alternately compressed and stretched the planet, heating it up into its current, hugely bloated form.

The WASP cameras detect exoplanets using the transit method, watching for the tell tale dip in stellar brightness as a candidate planet transits in front of the parent star.

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