Kamikaze Planet

Astronomers have found a giant planet orbiting so close to its parent star that it's bound to spiral inward to its doom or else be ripped to shreds by the star's gravity. Either way, the planet called WASP-18b should provide astronomers with a mother lode of data about the delicate gravitational balancing act that affects all solar systems.

Stars and their planets can influence one another in subtle but inevitable ways. Even though Jupiter orbits about 500 million kilometers away from our sun, for example, the planet's huge mass can shift the sun's center of gravity by more than 10,000 kilometers, enough to possibly reveal the planet's presence to alien astronomers. The moon's pull on Earth raises and lowers sea levels in the form of tides. And conversely, Earth's gravity has slowed the moon's rotation so that it always presents the same face to its parent planet.

Now researchers have found an extreme version of tidal forces at work. WASP-18b is roughly 10 times the mass of Jupiter, and it orbits only about 3 million kilometers from its star. WASP-18b, which was discovered with twin telescopes located in the Canary Islands and South Africa, is located about 400 light-years away in the constellation Phoenix.

WASP-18b and its star are so close to each other that it takes the planet, which is described in tomorrow's issue of Nature, less than a day to complete a revolution. As it orbits, WASP-18b drags bulges tens of kilometers high around the star's equator, making it spin faster. Those same forces also cost the planet its angular momentum, pulling it inward, eventually to the point where it will smash into the star's surface. Or perhaps it will be shredded to form a Saturn-like ring system. In either case, WASP-18b appears doomed to die within half a million years.

Or maybe not. Astronomer and co-author Andrew Collier Cameron of the University of St. Andrews in Fife, U.K., says there could be a third alternative: Some as-yet-unknown manifestation of tidal forces could prolong the planet's life by as much as 500 million years. That's because the close and fierce interaction between two large and essentially gaseous bodies could produce turbulence or other effects that could somehow cancel out WASP-18b's orbital deterioration. Astronomers may know whether that's happening within a decade or so, Collier Cameron says. By then, they will have collected enough data to determine whether WASP-18b's orbit has degraded and by how much. If WASP-18b somehow escapes imminent destruction by tidal forces, then astronomers will know that "our understanding of orbital dynamics, particularly tidal interactions, needs revision," says lead author Coel Hellier of Keele University in the United Kingdom.

"It's the find of a lifetime," says astronomer Douglas Hamilton of the University of Maryland, College Park. "The beauty of this discovery is that we will be able to know [how WASP-18b will meet its end] within 5 to 10 years," Hamilton says. "I think that it could go either way--which makes it very exciting."

By Phil Berardelli
ScienceNOW Daily News
26 August 2009


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