Discovery over 'warp speed' planets

Runaway "warp speed" planets could be flying through the galaxy and beyond at a sizeable fraction of the speed of light, scientists have said.

Computer simulations predict black holes acting like slingshots to rip planets from their orbits and fling them through space at colossal speed. A typical "hypervelocity planet" would travel at seven to 10 million miles per hour but some could reach speeds of up to 30 million miles per hour, a few per cent of the speed of light.

"These warp speed planets would be some of the fastest objects in our galaxy," said astrophysicist Dr Avi Loeb, from the Harvard-Smithsonian Centre for Astrophysics in Massachusetts, US. "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."

Astronomers already know that stars can be catapulted across space by black holes. The first runaway star was discovered seven years ago when astronomers spotted it flying out of our galaxy, the Milky Way, at 1.5 million miles per hour. Theorists then wondered if the same fate could befall planets.

In both cases, the phenomenon is caused by unimaginable gravitational forces near the super-massive black hole at the centre of the Milky Way.

A double star system wandering too close can be ripped apart. One star passes the point of no return and becomes captured by the black hole, while the other is ejected at high speed like a missile from a slingshot.

Scientists found that any planets orbiting the captured star could also be torn away and flung into interstellar space. Planets orbiting the ejected star would be carried along by their parent. A hypervelocity planet would eventually pass out of the Milky Way into the intergalactic void, the researchers believe.

"Other than subatomic particles, I don't know of anything leaving our galaxy as fast as these runaway planets," said Dr Idan Ginsburg, from Dartmouth College in New Hampshire, US.

Current instruments cannot detect a lone hypervelocity planet. But astronomers believe there might be a chance of spotting a "transiting" planet orbiting a hypervelocity star. The star's light would dim slightly as a tight-orbiting planet passed across its face.

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