Spin Cycle, Or You're Doing It Wrong

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

Except one.

(Where is Don LaFontaine when you need him...oh yeah, dead)

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. 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 but has not yet been published in a journal, but don't let that sway you - lots have things are still true and go unpublished. Like my lovemaking prowess.

As you recall from your 5th grade field trip to the planetarium, a star forms when a cloud of gas and dust collapses. This is not the same as when an fat person farts in their squalor. 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. Astronomers think our moon was made when our planet collided with a Mars-sized object. Earlier this week NASA's Spitzer Space Telescope found evidence of two planets colliding around a distant, young star. Apparently, they both wanted to take her out...you know how hot-headed males planets can be! 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.

WASP-17 is about half the mass of Jupiter but bloated to twice its size. The planet is only as dense as expanded polystyrene, 70 times less dense than Earth. This can be explained by a highly elliptical orbit, which brings it close to the star and then far away. Like our ocean tides, the "tidal effects" on WASP-17 heat and stretch the planet, the researchers suggest. Although you wouldn't want to do that if it really was made of polystyrene.