"Backward" Planet Has Density of Foam Coffee Cups

Rachel Kaufman
for National Geographic News
August 17, 2009

Contrary to a recent TV cell phone ad, Dunkin' Donuts isn't likely to set up shop in space any time soon.

But if it did, the donut chain might like to build next to WASP-17b, a newfound planet that's puffed up to be roughly as dense as a foam coffee cup.

WASP-17b orbits a star about a thousand light-years away. In addition to its exceptionally low density, the planet is one of the largest yet found.

"When I first saw that this thing might have a radius twice that of Jupiter, I was really astounded," said David Anderson of Keele University, a member of the U.K.-based Wide Area Search for Planets (WASP) consortium.

WASP-17b probably got so big because of its unusual orbit, Anderson and colleagues say in a new paper describing the find.

The planet is also the first found to orbit "backward" around its star, an eccentricity likely caused by a collision with a larger neighbor early in WASP-17b's life.

(Related: "Evidence of Huge Planetary Collision Found.")

That planetary crash may have nudged WASP-17b into an elongated orbit, which led to variations in the gravitational pull exerted on the planet by its host star, Anderson said.

Changes in the star's pull would have generated powerful tidal forces, which in turn would have created friction that got dissipated as heat. The planet's heated gases would have then expanded, causing the world to bloat.

But the astronomers haven't conclusively proven that WASP-17b has an eccentric orbit, Anderson said, and tidal forces aren't the only possible explanation for the planet's swollen size.

The gas giant could have expanded in a similar fashion, he noted, if the planet's early atmosphere was opaque enough to trap the right amount of heat from its star.

Findings published online August 11 in a paper submitted to the Astrophysical Journal.




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