Giant new planet orbits wrong way

Planet hunters have discovered the biggest new world yet and it is going the wrong way around its home sun. The find is unique and astronomers say the planet must have flipped direction after a near miss with another huge passing body swung it around like a slingshot.

The planet is also the biggest yet found at twice the size of mighty Jupiter. But it "weighs" only half as much as Jupiter and has a density like that of expanded polystyrene.

Astronomer David Anderson from Keele University, Staffs, picked out the bloated new world in collaboration with Amaury Triaud of Geneva Observatory, Switzerland, using observational data from South Africa.

They have dubbed the planet WASP-17 because it was the 17th planet beyond the solar system to be discovered by the Wide Area Search for Planets - a fantastically successful project run by a consortium of UK universities. They run two banks of sky cameras that were built with a bit of help from eBay.

WASP-17 is orbiting a star with the telephone-number label of USNO-B1.0 0619-0419495, lying around 1,000 light-years away in Scorpius. It should be travelling in the same direction that the star is spinning, like every other planet known does. Instead it has what astronomers call a retrograde orbit.

The astronomers put this down to a form of cosmic billiards when planets in newly forming solar systems often had close encounters and even collisions. The discovery, reported in the Astrophysical Journal, is said to cast new light on how planetary systems form from disks of gas and dust and evolve.

Professor Coel Hellier, of Keele University, waxed poetic to comment: "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."

David Anderson said: "Newly formed solar systems can be violent places. Our own Moon is thought to have been created when a Mars-sized planet collided with the recently formed Earth and threw up a cloud of debris. 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."

But why is WASP-17 so big? The discovery team suggests that have been subjected to intense tides as it travelled in its strange and highly-elliptical orbit, causing it to become stretched and bloated.

Professor Hellier said: "This planet is only as dense as expanded polystyrene, 70 times less dense than the planet we're standing on."

WASP - sometimes called SuperWASP - works by monitoring hundreds of thousands of stars. The cameras watch for tiny fades in starlight caused when a planet passes in front of the star in a transit. The same transit technique has allowed astronomers to analyse the atmospheres of extrasolar planets.

British scientists assembling the instruments, now operating in South Africa and on the Canary Island of La Palma, had to turn to eBay to track down lenses after they learned that Canon had stopped making them.

Picture: An artist's impression of two close extrasolar planets (Credit: KASI/CBNU/ARCSEC).

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