Backward planets may have flipped into place

Reverse orbits could make Earthlike neighbors unlikely

A bevy of backward-orbiting exoplanets could challenge theories of planet formation, new research suggests. The planets’ wonky orbits might also rule out the presence of Earthlike bodies in some planetary systems.

COMMON DIFFERENCE A suite of new exoplanets that orbit their stars backward shows that these bodies aren’t so unusual. ESO/L. Calcada

The wrong-way planets got where they are by cartwheeling over their stars’ heads, Andrew Collier Cameron of the University of St Andrews in Scotland proposed in an April 13 presentation at the Royal Astronomical Society’s National Astronomy Meeting in Glasgow, Scotland.

Planets are thought to form from the disk of gas and dust that surrounds a young star. Because the star and the disk both coalesce from the same cloud of material, theory holds that both should spin in the same direction — and so should any planets that arise. The “disk migration theory” posits that some planets should end up close to their stars by gently migrating inward over time, maintaining an orbital plane in line with the star’s rotation.

Last summer, astronomers first discovered a handful of planets that threw that idea for a loop. These planets orbit backward, opposite to the direction of their stars’ spin (SN: 9/12/09, p. 12). And other newly discovered planets that did have “forward” orbits were tilted 20 degrees or more with respect to the plane of the stellar disk where they were born.

These planets belong to a class of extrasolar planets called hot Jupiters — giants that sit scorchingly close to their stars.

 “If I had to stick my neck out and make a prediction, it’s probably not a good idea to go looking for terrestrial planets in systems that have hot Jupiters in them,” Cameron says.

Cameron and his colleagues think a single mechanism pushed the tilted and backwards planets into their offbeat orbits and also drew them close to their stars. If these slanted orbits are common, it could be a death knell for the migration theory, says study coauthor Didier Queloz of the Geneva Observatory.

“Migration cannot produce misaligned systems,” Queloz says. The new study brings the total number of planets for which astronomers have angle data up to 27. Of those many are misaligned, with half tilted at steep angles and six orbiting backwards.

“Since most hot Jupiters are indeed misaligned, most cannot be formed by migrations,” Queloz says. “We’re kind of killing this first idea of migration.”

The more likely explanation, the researchers say, is the Kozai mechanism. In this scenario, a second, distant large body like a planet or a companion star gravitationally perturbs a planet’s orbit. The orbital plane can flip over the top of the star like a jump rope. When the orbit is flipped more than 90 degrees, the planet is orbiting backwards. At the same time, the shape of the orbit squishes and stretches like a rubber band. As the planet gets closer to the star, its orbit gets more circular, and the cartwheels become less dramatic. When the orbit finally settles into a circle near the star the tilt freezes.

Earlier research predicted that most orbits of giant planets perturbed by the Kozai mechanism should end up tilted around either 40 degrees — a forward but slanted orbit — or 140 degrees — a backwards orbit.

“That looks very much like what we’re now observing,” Cameron says. “It looks almost too good to be true.”

Some critics think he’s right — it is too good to be true. “I think they’re eliminating the standard mechanism of disk migration prematurely,” says Adam Burrows of Princeton University. Some combination of migration, scatter and the Kozai mechanism is still possible, he says. “Their data isn’t that definitive to eliminate any other possibilities.”

Astronomers had hoped that smaller, more Earthlike planets could be hiding in the neighborhoods of hot Jupiters, but the recent slug of orbital data suggests that may be unlikely. The giant planets’ orbits can take hundreds of thousands of years to settle, “during which you have a rampaging Jupiter on a cometlike crazy tumbling orbit, which would simply fling any remaining debris out of the system,” Cameron says.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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