'Hypervelocity Planets' Zoom Through the Galaxy at 30 Million Miles Per Hour

Runaway Planet!

David A. Aguilar (CfA)

Astronomers have long known that when binary star systems wander too close to a supermassive black hole under the right conditions, they can be torn apart in such a way that one star is pulled into orbit around the black hole and the other is violently ejected outward, sending it speeding out of the galaxy and into interstellar space. Now it turns out individual planets can suffer a similar fate--and when they do, they can do so at up to 30 million miles per hour, making them some of the fastest-moving objects in our galaxy.

New research coming out of the Harvard-Smithsonian Center for Astrophysics and Dartmouth College shows that these "hypervelocity planets" are the fastest moving objects leaving our galaxy with the exception of subatomic particles. Typically they would be traveling 7 to 10 million miles per hour, but under certain conditions they might be spun out of the galaxy at much higher rates of speed.

The planets get kicked out of the galaxy when their stars do. The team modeled a binary star system approaching a black hole, with each star hosting a couple of planets. They found that in an instance where the star system is torn apart and one star is kicked outward, that star can take its planets with it. They also found that planets orbiting the star that doesn't get booted from the galaxy still might be jettisoned from the galactic center at high speed.

Astronomers still haven't seen one of these planets, because they would be dim and moving really, really fast. But if a planet started out in a very close orbit to its star and its star got the boot, it could remain in orbit around that star even as the star makes a beeline for intergalactic space. That means it's feasible that astronomers could spot the planet transiting its star, the same way they spot exoplanets orbiting other stars in the galaxy.