Newfound Planet Orbits Backward

Planets orbit stars in the same direction that the stars rotate. They all do. Except one.

A newfound planet orbits the wrong way, backward compared to the rotation of its host star. Its discoverers think a near-collision may have created the retrograde orbit, as it is called.

The star and its planet, WASP-17, are about 1,000 light-years away. The setup was found by the UK's Wide Area Search for Planets (WASP) project in collaboration with Geneva Observatory. The discovery was announced today but has not yet been published in a journal.

A star forms when a cloud of gas and dust collapses. Whatever movement the cloud had becomes intensified as it condenses, determining the rotational direction of the star. How planets form is less certain. They are, however, known to develop out of the leftover, typically disk-shaped mass of gas and dust that swirls around a newborn star, so whatever direction that material is moving, which is the direction of the star's rotation, becomes the direction of the planet's orbit.

WASP-17 likely had a close encounter with a larger planet, and the gravitational interaction acted like a slingshot to put WASP-17 on its odd course, the astronomers figure.

Cosmic collisions are not uncommon. Earth's moon was made when our planet collided with a Mars-sized object, astronomers think. And earlier this week NASA's Spitzer Space Telescope found evidence of two planets colliding around a distant, young star. Some moons in our solar system are on retrograde orbits, perhaps at least in some cases because they were flying through space alone and then captured; that's thought to be the case with Neptune's large moon Triton.

The find was made by graduate students David Anderson at Keele University and Amaury Triaud of the Geneva Observatory.

WASP-17 is about half the mass of Jupiter but bloated to twice its size. That can be explained by a highly elliptical orbit, which brings it close to the star and then far away. Like exaggerated tides on Earth, the tidal effects on WASP-17 heat and stretch the planet, the researchers suggest.

"This planet is only as dense as expanded polystyrene, 70 times less dense than the planet we're standing on," said professor Coel Hellier of Keele University.

WASP-17 is the 17th extrasolar planet found by the WASP project, which monitors hundreds of thousands of stars, watching for small dips in their light when a planet transits in front of them. NASA's Kepler space observatory is using the same technique to search for Earth-like worlds.