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An illustration of Kepler-16b, a planet orbiting two stars.

An artist's concept shows Kepler-16b, the first planet known to orbit two stars.

Illustration courtesy T. Pyle, Caltech/NASA

Andrew Fazekas

for National Geographic News

Published February 6, 2012

A planet in a two-star system can end up in a gravitational ping-pong match that can last for millenia before the planet gets ejected into interstellar space, a new study suggests.

Scientists had previously theorized that gravitational interactions among multiple planets orbiting a star can sometimes cause a world to get ejected from its system, leaving the rogue planet to wander alone.

(Related: "Earth-Size 'Lone Wolf' Planets May Host Life.")

Now a complex set of computer simulations shows that certain types of binary star systems might not let go of wayward worlds so easily.

Instead, when a planet gets tossed out by its binary parent, the world can bounce over to the stellar companion. The hapless planet then begins to orbit wildly, only to end up being tossed back to the original star.

(Watch animations of planets bouncing between binary stars.)

This gravitational bouncing can go on for as long as a million years, until ultimately the planet gets flung completely out of the binary system.

"The trigger for the bouncing is the close approach of another planet" also orbiting the initial star, said study co-author Dimitri Veras, an astronomer with the U.K.'s University of Cambridge.

"Once the bouncing has begun, the [ejected] planet is moving too fast to settle into a long-term stable orbit around either star."

Bouncing World Would Burn, Then Freeze

According to the new models, the bouncing effect is more likely to occur when the separation between the binary stars is anywhere from 6 to 25 times the distance between the sun and Pluto, which is 3.6 billion miles (5.9 billion kilometers), on average.

Any closer and planets can have stable orbits around both stars—just like the "Tattoine" planets being discovered by NASA's Kepler spacecraft.

"Because 25 times the sun-Pluto distance is about equal to the maximum separation of binary stars in the Milky Way, we deduce that most types of binary star systems can have bouncing if the planets are in the correct locations," Veras said.

In their simulations, Veras and colleagues examined binary star systems in which at least one of the stars has multiple orbiting planets.

Gravitational bouncing can start at any point in the evolution of a planetary system, according to the new models.

Perhaps not surprisingly, though, the habitability of any "ping-pong" planets would be compromised due to the wildly erratic temperatures the world would experience as it moved between its two parent stars.

(Related: "How Planets Can Survive a Supernova.")

In most cases, the bouncing planet would have various, highly eccentric orbits, which means surface temperatures would drastically oscillate from hot to cold.

And if the planet starts out with an atmosphere similar to Earth's, the alien world would also be subject to a runaway greenhouse effect whenever the planet made a close approach to one of the stars.

Ping-Pong Planets a Rare Breed?

The idea of objects being transferred between two celestial bodies is not new and has been understood for a number of years, said Matthew Holman, a senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics who was not connected to the study.

For example, "this effect has been proposed as a cheap way to transfer [spacecraft] between near-Earth orbit and near-moon orbit," Holman said.

Given the latest findings, planet-hunting astronomers might now ask just how common bouncing planets are across the galaxy.

(Related: "'Solar Systems' Common Across the Galaxy, NASA Probe Hints.")

"I would think that such systems would be exceedingly rare, since they require special orbital configurations," such as the necessary distances between the stars and the presence of multiple planets orbiting one star in the pair, Holman said.

Also, bouncing planets may be tough to confirm, because they bounce for only a short time, relatively speaking, he added.

Earth, for instance, has been stably orbiting the sun for roughly 4.5 billion years, while it would take just a million years or so before a bouncing planet "would either escape through an interaction with another planet or ... collide with one of the stars."

The bouncing-planet study was posted last week on the research website arXiv.org and has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

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