Runaway planets ejected from galaxy at insane speeds

Planets in tight orbits around stars that get ejected from our galaxy may actually themselves be tossed out of the Milky Way at blisteringly fast speeds of up to 30 million miles per hour, or a fraction of the speed of light, a new study finds.

"These warp-speed planets would be some of the fastest objects in the galaxy, aside from photons and particles like cosmic rays," said Avi Loeb, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "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."

In 2005, astronomers found evidence of a runaway star that was flying out of the Milky Way galaxy at a speed of 1.5 million mph (2.4 million kph). This hypervelocity star was part of a double-star system that wandered too close to the supermassive black hole at the center of the galaxy.

The strong gravitational pull at the galactic center ripped the stars apart, sending one hurtling through space at high speeds, while capturing the other to stay in orbit around the massive black hole.

In the seven years since, 16 of these hypervelocity stars have been found, and Loeb and his colleagues began to wonder whether planets could also be sent tearing through space at such extreme velocities.

Cosmic pinball

The researchers created simulations that examined what would happen if each star had at least one planet orbiting nearby. They found that up to 10 percent of planets tightly orbiting one of the stars could stay bound as the star is flung outward. The other star that is captured by the black hole could also have its planet ripped away from it, and this planet would then be pitched into interstellar space at intensely fast speeds as well.

"We were trying to predict, if you have planets around each of the stars in the binary system, what fraction of the planets might go along with the hypervelocity star for the ride," Loeb told SPACE.com. "What we found is that some of them get expelled at high speeds, up to a few percent of the speed of light. Some of the planets get ripped apart from the host stars and get sent out at high speeds, and they also become hypervelocity planets this way." 

These planets would travel through space at unparalleled velocities, the researchers said.

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