Oddball Planet Goes the Wrong Way & Is Dense as Packing Peanuts

By Amos Zeeberg (Discover Web Editor) | August 12, 2009 6:25 pm

planetsScientists have spied a new exoplanet–and not only is it the biggest one yet, but it’s also moving the wrong direction. Unlike other planets, which orbit the same way their stars rotate, WASP-17 moves the opposite way, according to a study published in Astrophysical Journal.

Planets are born from the same ball of rotating gas that creates their parent star, which is why they usually orbit — and spin — in the same direction as their star. While WASP-17 is the first planet known to orbit backwards, some planets in our own solar system, such as Venus, are spinning backwards [Wired]. WASP-17’s backwards motion is known by astronomers as a retrograde orbit.

The new planet, which is 1,000 light-years from Earth and, may have been thrown onto its odd course from a collision in a form of cosmic billiards, when planets in newly forming solar systems often had close encounters and even collisions…. “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” [Skymania News], said co-author David Anderson.

It seems that WASP-17’s two most-defining traits–its size and odd orbit–may be linked. [S]cientists think the planet’s highly elliptical… orbit might have created extra-strong terrestrial tides, which cause planets to expand and contract. The constant stretching could have inflated WASP-17 to its current swollen size [Wired]. Indeed, the planet is about 70 times less dense than Earth–about as compact as expanded polystyrene, the material that comprises packing peanuts.

The planet is the 17th to be discovered by the Wide Area Search for Planets, which is run by a group of universities in the U.K. The project’s cameras monitor hundreds of thousands of stars, watching 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 [Skymania News]. This discovery could help scientists learn more about how systems of planets stem from gas and dust and change with time.

Related Content:
80beats: NASA’s New Kepler Spacecraft Is Ready to Find Some Earths
80beats: Will NASA’s Next Step Be an Astronaut Rendezvous With an Asteroid?
80beats: World’s Biggest Telescope Will Provide “Baby Pictures” of the Universe

Photo courtesy of KASI/CBNU/ARCSEC


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