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'Runaway planets' hurled from galaxies at 30 million mph

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In this artist's conception, a runaway planet zooms through interstellar space. New research suggests that the supermassive black hole at our galaxy's center can fling planets outward at relativistic speeds. Eventually, such worlds will escape the Milky Way and travel through the lonely intergalactic void.David A. Aguilar (CfA)

Now that’s a wild ride!

Astronomers have discovered that the incredible gravitational strength of supermassive black holes can tear planets away from their star systems and hurl them through space at incredible speeds -- as fast as 30 million mph.

They noted that this is "a few percent" of the speed of light, a theoretical constant of 186,000 miles per second or about 670 million mph.

Called hypervelocity planets, the speedy worlds vastly outrace runaway stars that scientists found flying out of our galaxy seven years ago at the tortoise-like pace of just 1.5 million miles per hour.

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

"You'd be in for a wild ride from the center of the galaxy to the Universe at large."

- Astrophysicist Avi Loeb

For this study, the researchers simulated a double-star system that wanders too close to the supermassive black hole at the galactic center. They had already known that the black hole’s gravitational forces could rip the stars apart -- sending one away at high speed while the other is captured into orbit around the black hole.

But what would happen if each star had a planet or two orbiting nearby?

The researchers found that the star ejected outward could carry its planets along for the ride. The second star, as it's captured by the black hole, could have its planets torn away and flung into the icy blackness of interstellar space at tremendous speeds.

A typical hypervelocity planet would slingshot outward at 7 to 10 million miles per hour. However, a small fraction of them could gain much higher speeds under ideal conditions.

"Other than subatomic particles, I don't know of anything leaving our galaxy as fast as these runaway planets," added lead author Idan Ginsburg of Dartmouth College.

Current instruments can't detect a lone hypervelocity planet since they are dim, distant, and very rare. However, astronomers could spot a planet orbiting a hypervelocity star by watching for the star to dim slightly when the planet crosses its face in a transit.

For a hypervelocity star to carry a planet with it, that planet would have to be in a tight orbit. Therefore, the chances of seeing a transit would be relatively high, around 50 percent.

"With one-in-two odds of seeing a transit, if a hypervelocity star had a planet, it makes a lot of sense to watch for them," said Ginsburg.

Eventually, such worlds will escape the Milky Way and travel through the intergalactic void.

"Travel agencies advertising journeys on hypervelocity planets might appeal to particularly adventurous individuals," added Loeb.