Planets hurtling near the speed of light? It's possible, study says.

The thinking: A flare from a doomed sun-like star would happen about once every 10,000 years and last for months, noted a team of astronomers from the University of Leicester in Britain and the University of Amsterdam in the Netherlands last fall. But observers had detected daily flares that would last for a few hours. 

Earlier this year, researchers discovered what they interpreted as a gas cloud falling into the black hole. One possible explanation: The cloud originally orbited a low-mass star, but got yanked free to feed the black hole. The presence of a gas cloud near the galactic core would also suggest that planetary building-blocks exist there. 

But what would happen to planetary systems after a close encounter with a supermassive black hole? 

Loeb and collegues considered planets orbiting binary stars at a distance of 2,000 Astronomical Units from the galaxy's central black hole – or about 2,000 times the distance from Earth to the sun. If the stars contained one or two Jupiter-scale planets each, gravitational interactions between the stars and the black hole could send one star and its offspring hurtling out of the galaxy.

The remaining star would be stripped of its planets, which also could be ejected. Under the right conditions, the team calculated, these starless planets could leave the galaxy at a speed a few percent of the speed of light. Meanwhile, the now-planetless star left behind would migrate closer to the black hole.

“Other than subatomic particles, I don't know of anything leaving our galaxy as fast” as the fastest runaway planets in the team's simulations, said Idan Ginsburg, a graduate student at Dartmouth College in Hanover, N.H., and the new paper's lead author, in a prepared statement.

For a planet to remain bound to an ejected star, it would have to be orbiting very close to the star, Loeb says. That also would significantly boost the chance that big, ground-based telescopes could spot the effect these planets would have on the star's light as they transit their sun.

If hypervelocity planets or planetary systems exist, and if they harbored life sentient enough to notice, “this ride would be quite exciting,” Loeb says.

A planet system would move slower than an unbound planet, but in principle it could still be in for the ride of its life, he adds. It could get swept up into the accelerating expansion of space itself, he speculates, eventually reaching velocities that from an Earth-bound vantage point would appear to approach the speed of light.

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