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Planets Are Being Shot at 'Warp-Speed' Out of the Milky Way

Norman Byrd's picture

A new study suggests that not only could there be planets encircling fast-moving -- "hypervelocity" -- stars flung away from the galaxy's central black hole, but there also could be unattended planets hurled toward the Milky Way's edge at millions of miles per hour.

Free-roaming or "rogue" planets within the Milky Way have been a known commodity to scientists for some time, but astronomers studying stars on a trajectory to leave the galaxy now believe that there is enough evidence to suggest that those stars just might be accompanied by planets. They're not exactly rogue planets due to their relationship with a parent star, but they're certainly free-roaming if taken as a set with said star. And those planets, according to the researchers, are traveling at "warp speed."

Astrophysicist Avi Loeb of Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., said, according to, "These warp-speed planets would be some of the fastest objects in the galaxy. In terms of large, solid objects, they would be the fastest. It would take them 10 seconds or so to cross the diameter of the Earth."

Simulations created by the researchers were created to examine the effects of the gravitational pull of a supermassive black hole on binary stars that had at least one planet each in orbit. The study found that at least 10 percent of planets orbiting stars would remain with the parent star. The rest would either be captured by the black hole or be flung away at enormous "warp" speeds.

Researchers studying escaping stars found that it was quite possible that worlds holding a tight orbital path around such a parent star just might go along for the ride. And just how fast would they be traveling? Some as ast as their parent star, all things being relative.

But it is believed that the unattending fast-moving worlds could reach speeds of up to 30 million miles per hour.

At present, there are 16 stars -- called "hypervelocity" stars -- that have been discovered on trajectories headed toward intergalactic space. The first was discovered in 2005 and it was found to be traveling at 1.5 million miles per hour and headed toward the galaxy's edge. It was believed to have once been part of a binary star system that had gotten a bit too close to the supermassive black hole at the galactic core. Where one of the stars might have been captured and remains in orbit around the galactic center, the other was discharged at tremendous speeds by the gravitational pull of the black hole and its separating effect on both stars.

Lead author of the study, Idan Ginsburg of Dartmouth College, said in a statement: "Other than subatomic particles, I don't know of anything leaving our galaxy as fast as these runaway planets."

Loeb noted that some of the planets would also be ripped away from their parent stars, thus becoming hypervelocity planets on their own, with velocities far exceeding other runaway planets.

Astronomers are hoping that the study will call attention to the hypervelocity stars and prompt other studies to look for companion planets. In fact, Loeb stated that the purpose of the study was to propose just that. With the technology available in some existing large telescopes, planets that pass across the face of their parent stars -- in what is called a "transit" -- might be detectable.

Ginsburg said that the odds of finding such a tight-orbit world were one-in-two.

There could also be quite a few more of these "warp-speed" planets and hypervelocity stars. It is estimated that a full 1/3 of all the stars in the Milky Way belong to binary or multiple-star arrangements.

A study using gravitational microlensing released in May 2011by Takahiro Sumi, an astrophysicist at Osaka University (Japan), noted that the number of "rogue" or free-roaming planets within the galaxy could be quite enormous -- as many as 400 billion. Those rogue worlds, however, were believed to have been lost or ejected by their parent stars.

Hypervelocity planets are a different variety of rogue altogether, due to both their creation and speed.

The hypervelocity planet study is scheduled for publication in an upcoming edition of the Monthly Notices of the Royal Astronomical Society.

(photo credit: NASA, ESA and G. Bacon, Wikimedia Commons)

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