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When Runaway Planets Go 30 Million Miles Per Hour

A new discovery indicates some planets may be flung out of our galaxy at velocities a few percent of the speed of light

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An artist's conception of a runaway hypervelocity planet. Image courtesy of David Aguilar, Harvard-Smithsonian Center for Astrophysics

In 2005, Warren Brown of the Smithsonian Astrophysical Observatory noticed something rather unusual in the sky: a star traveling out of the Milky Way galaxy at roughly 1.5 million miles per hour. The strange discovery could only be explained by an even stranger prediction, made nearly two decades earlier by an astronomer named J.G. Hills.

“He predicted that if you have two stars orbiting each other—a so-called binary system—and they get too close to the central black hole in the Milky Way, they will get ripped apart,” says SAO astrophysicist Avi Loeb. “One of the stars will go into a tighter orbit around the black hole, and the second one will be flung out of the galaxy.”

Since Brown’s 2005 discovery, at least 21 hypervelocity stars (as they’ve come to be called) have been observed speeding out of our galaxy. But only recently did anyone look to see if there might be hypervelocity planets, as well. “My collaborator Idan Ginsburg and I did some work on hypervelocity stars, and at some point, I was talking with him about perhaps looking into planets,” Loeb says. “One day, at lunch, it clicked: we could actually write a paper on them, because there is a method of finding them.”

Loeb had realized that a planet orbiting one of these hypervelocity stars could be observed by what’s called the transit method: when a distant planet crosses between its star and our telescope, the light of the star dims slightly, indicating the presence of the planet. First, though, he and Ginsburg had to determine whether these planets could theoretically exist in the first place. Their calculations, published last week in the Monthly Notices of the Royal Astronomical Society, went beyond even what he had suspected.

Hypervelocity planets can indeed exist—and according to the research team’s simulations, they may approach speeds as high as 30 million miles per hour, making them some of the fastest-moving objects in the known universe.

“We asked what would happen if there were planets around hypervelocity stars,” Loeb says. “So we started with a simulation of a binary system, and then sprinkled planets around each of the stars.” Their calculations showed that, if the binary star system was ripped apart by gravitational forces near the galaxy’s central black hole, a small percentage of the planets would stay bound to one of the stars, either following them on their journey out of the galaxy, or diving more closely into the depths of the black hole. The majority of planets, however, would be flung away from their parent stars, traveling even faster to the edges of the Milky Way.

“Their speed can reach up to ten thousands kilometers per second—a few percent of the speed of light,” says Loeb. “If you imagine a civilization living on such a planet, they would have a tremendous journey.” The voyage from the center of the galaxy to the edge of the observable universe, he says, would take 10 billion years.

The potential existence of hypervelocity planets is far more than a mere curiosity, since it would provide us information about conditions near the center of the galaxy, and if planets can even form there. “It’s a very unusual environment, because the density of stars there is more than a million times than the density near the sun,” Loeb says. “There is a very high temperature, and every now and then the black hole at the center gets fed with gas, so it shines very brightly, which could in principle disrupt a system that tries to make planets.” His team’s calculations showed that, if planets can indeed form in this area, they should be observable when bound to hypervelocity stars.

None of these planets has been spotted, but Loeb hopes that some will be found in coming years. Just as astronomers have recently discovered hundreds of extrasolar planets using the transit method as part of NASA’s Kepler Mission, they can scrutinize hypervelocity stars in much the same way to spot these runaway planets. And if things progress along the same time frame as J.G. Hills’ 1988 prediction of hypervelocity stars, Loeb can expect to have his predictions confirmed within his lifetime—sometime around the year 2029.

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