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Giant new planet's
backward orbit


Posted: August 12, 2009

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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.

A close encounter with a passing object may have spun WASP-17 around in its tracks. Image: KASI/CBNU/ARCSEC.

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.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!


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