Oops! This planet is orbiting in the wrong direction

If astronomers have any notion of giving their newly discovered planet, WASP-17b, a more decent-sounding name, they should consider naming it after Wrong Way Corrigan.

The crafty Mr. Corrigan was a pilot from California who took off from an airport in Brooklyn, N.Y., on a foggy July evening in 1938. He claimed he was heading back to California. He wound up in Dublin, Ireland, 28 hours and 13 minutes later. (He told grumpy federal officials that he misread his compass in the fog; he actually headed east on purpose. But that's another story.)

Anyway, the new planet in question – about 1,000 light-years away – appears to be orbiting its parent star in the wrong direction. This is the first navigationally challenged planet anyone has seen. You can download a PDF of the formal report of the discovery here. A plain-English version appears here.

So, how can there be a "wrong" direction? This is outer space, after all. Oh, it's way wrong!

Researchers explain that when a star and its solar system form, they do so from the same rotating cloud of dust and gas. Everything that forms – from the star itself to the objects that orbit it – initially spins in the direction in which the cloud was rotating. The objects orbiting that star also orbit in the direction of the cloud's original rotation.

But the star WASP-17 is spinning in one direction. WASP-17b is orbiting in the opposite direction. That means that somewhere along the way, the latter was yanked in the opposite direction, either by a close encounter with another planet or perhaps a passing star.

Astronomers have found no evidence yet for a second planet, says Coel Hellier, an astronomer at Keele University in Britain and a leading member of the team reporting the discovery.

In an e-mail he explains that a second planet could still be orbiting the star, but at a much greater distance, making it harder to detect. Or the encounter could have sent the second planet packing – ejecting it from the system.

Based on models of the evolution of orbits, he estimates that the orbit-reversing event took place about 1 billion years ago. Based on the studies of the system's star, the researchers estimate that the system is about 3 billion years old.

WASP-17b is intriguing for other reasons. It has half Jupiter's mass, but is twice Jupiter's size. It orbits WASP-17 once every 3.7 Earth days.

The planet's backwards orbit could help explain its girth, the research team says. After the encounter, it would have been knocked into a highly elliptical orbit. The constant squeezing and stretching of WASP-17b by the star's gravity as the planet approached and left the star's immediate vicinity probably heated the planet. Over time, the orbit would have grown increasingly circular, leading to the orbit that astronomers see today.

Under such gravitational tugging and hauling, WASP-17b expanded to the point where its density is now about the same as Styrofoam peanuts used for boxing up breakables.