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Backward planets rewrite book on orbits

Lisa Grossman

Two exoplanets that orbit their star backwards have been found. The first planet is also the most bloated found to date, and some astronomers suspect both properties can be traced back to a close encounter with a planetary sibling.

The solar system generally rotates like a record album, with most objects orbiting the sun in the same direction as the sun itself spins. This is thought to result from the fact that everything formed from the same natal cloud of gas and dust.

However, some objects, including a number of comets and asteroids, move in orbits that are so tilted with respect to the orbital plane of the planets that they end up travelling in the opposite direction. Astronomers think they were gravitationally thrown out of their original orbits by passing objects.

Three extrasolar planets have previously been found on orbits that are tilted with respect to the plane they should have been born in. But now, a planet called WASP-17b has been found on an orbit that is so tilted - by about 150° - that it is moving in the opposite direction to the spin of its star.

"All the others have been going in more or less the right direction, just tilted at crazy angles," says team member Andrew Collier Cameron of the University of St. Andrews in Scotland. WASP-17b, which lies 1000 light years away from Earth, is "pretty weird", he told New Scientist. "It really throws the cat among the pigeons."

Close call

The researchers, led by David Anderson of Keele University in the UK, found the planet by observing how it dimmed the light of its host star as it passed in front of, or transited, the star. They made the observations using a telescope array at the South African Astronomical Observatory.

Follow-up observations showed the direction of the star's spin - half of the star appears to move toward us, and half away from us. 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. But the researchers found that the opposite was true.

It may have been thrown onto the strange path after a near-collision with another, as-yet-undetected, planet in the same system. "A near-collision with the right trajectories can make a gravitational slingshot that flings one of the planets into a retrograde orbit," says team member Coel Hellier of Keele University.

Floating planet

WASP-17b is also a contender for the largest exoplanet ever found. It is about half the mass of Jupiter but up to twice its width, possibly de-throning the previous largest-known exoplanet. It's about as dense as polystyrene, and if you could place it in a bathtub, it would bob like a beach ball. It is only 7 million kilometres away from its star - eight times as close as Mercury is to the sun, and orbits once every 3.7 days.

Astronomers have found several similarly puffy planets in recent years, and have been at a loss to explain them. WASP-17b's backward dance may provide a clue.

Planets that are "scattered" by gravitational interactions tend to be thrown onto elongated, rather than circular, paths. And planets that are on elongated paths - particularly those that come very close to their stars, as WASP-17b does - feel strong gravitational tugs called tidal forces.

The planet feels the star's gravity at different strengths along its orbit, and this varying pull causes the planet's body to deform, generating heat that can cause the planet to puff up. "It doesn't require too much of this sort of effect to give you radii similar to Jupiter's," says Adam Burrows of Princeton University, who was not part of the new study.

Cartwheeling motion

Hellier says the scattering explanation can account for the team's observations: "WASP-17 comes along and fits very well in the theory, and provides an actual planet that is behaving as these theories predicted."

But not all team members are convinced. Collier Cameron, whose group wrote the planet-detection software, is sceptical that a planet could be gravitationally scattered into such a tilted orbit. "The theorists are going to have to work really hard to balance the books on this one," he told New Scientist.

Instead, he thinks the host star could have a smaller, unseen companion, whose gravity causes the plane of the planet's orbit to change over time. The planet's orbit would sometimes be around the star's equator, then it would gradually tilt so that it was over the poles, continuing in this cartwheeling motion, which is known as the Kozai mechanism.

Chance observation?

If that is the case, researchers observing the planet at a different time - say, millions of years from now - could see it moving in the same direction as its host star spins.

"The orbit is gradually flipping over the pole of the star," Collier Cameron says. "You've got a 50-50 chance of seeing them in prograde or in retrograde, depending on when you happen to look at them."

Planets on tilted orbits may be common. Fewer than two dozen planets have had their orbital planes measured, and already four have so far been found to orbit at unusual angles. "We're just starting to scratch the surface of these objects, but clearly they're out there, and in fairly large numbers," Collier Cameron says.

But it's not yet clear whether gravitational scattering by other planets or the influence of another star is to blame. "Each of these theories is possible - you can't really eliminate one or the other," Burrows told New Scientist. "It's really an open question. We may be slow in figuring out what actually makes more sense."

Journal reference: Paper submitted to the Astrophysical Journal


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