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Warp-speed planets ejected from Milky Way
Posted: 23 March 2012

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New simulations suggest that planets, just like stars, could be booted out of their home galaxies at neck-breaking speeds of 30 million miles per hour.

"We definitely see hypervelocity stars out there, and it's no stretch of the imagination to believe that there are hypervelocity planets out there as well," says lead author of the study Idan Ginsburg of Dartmouth College.

An artist impression of a runaway planet leaving its home galaxy behind. New research suggests that the supermassive black hole at our Galaxy’s heart can fling planets clean out of the Milky Way. Image: David A. Aguilar (CfA).

The first hypervelocity star was identified in 2005, an outcast star booted at around two million miles per hour out of our home Galaxy by its own black hole. Just as these runaway stars get an unwelcome kick from a supermassive black hole, if planets happen to be orbiting that star then they are in for a rough ride, too.

"These warp-speed planets would be some of the fastest objects in our Galaxy," says astrophysicist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics. "If you lived on one of them, you'd be in for a wild ride from the centre of the Galaxy to the Universe at large!"

Simulating a binary star system, each star with at least one planet, the scientists found that while one stellar system is captured into orbit around the black hole, the other is catapulted with its planets out into space.

"In terms of hypervelocity planets, our simulations showed that they occurred in roughly 40 percent of cases for binary star systems with two stars, and over 70 percent for systems with four stars," Ginsburg tells Astronomy Now. "These assume that you have a planet very close to the star, and the star is tidally disrupted by the massive black hole."

On average these planets would be ejected at speeds of 7-10 million miles per hour, but a small fraction could attain speeds of 30 million miles per hour, a few percent of the speed of light.

The planets' proximity to their host star would make transits the likely method of their detection, the star's temporary dip in brightness as a planet passes in front of it giving its presence away before careering out of the Galaxy.

The research will be published in the Monthly Notices of the Royal Astronomical Society.