Stargazers spy retrograde planetary bloater

Astronomers have spotted what they claim is the first exoplanet in a retrograde orbit around a star - a bloated body which is also the "least-dense planet currently known".

Dubbed WASP-17b, the body measures 1.5-2 Jupiter radii but weighs in at just half its mass, meaning it's about a dense as polystyrene and "if you could place it in a bathtub, it would bob like a beach ball", as New Scientist nicely puts it.

It's orbiting its star every 3.7 days at a distance of just 7 million kilometres, or "eight times as close as Mercury is to the sun".

What's got the stargazer's excited, though, is that WASP-17b is orbiting in the opposite direction to the star's spin, at an angle of 150 degrees with respect to the plane in which it was born from the cloud of dust and gas which formed the system.

WASP-17b was identified by researchers led by David Anderson of the UK's Keele University, who observed from the South African Astronomical Observatory how it dimmed the star's light as it transited.

Further observations revealed the retrograde orbit. New Scientist elaborates: "If the planet had orbited the star in the same direction as the star spins, it would have blocked some of the light from the approaching side first." The team, however, found the reverse was true.

Three other exoplanets with severely tilted orbits have been discovered, but WASP-17b is the first showing backwards tendencies.

Quite what caused this aberration is unclear. The scientists suggest it may be the result of a near miss with another planet in the system. Team member Coel Hellier of Keele University offered: "A near-collision with the right trajectories can make a gravitational slingshot that flings one of the planets into a retrograde orbit."

Regarding the planet's size, Adam Burrows of Princeton University suggested to New Scientist that tidal forces may be the cause, prompting WASP-17b to "puff up" under the influence of the star's gravity. He said: "It doesn't require too much of this sort of effect to give you radii similar to Jupiter's."

New Scientist has further info here, and the team's findings are published in the Astrophysical Journal here. ®