Black Hole Punch Can Launch a Planet

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Punched by the gravitational fist of Sagittarius A* (Sgr A*), the location of the supermassive black hole that lives at the center of our galaxy, a planet could be hurled through space at speeds of up to 30 million miles per hour (48 million kilometers per hour), the Harvard-Smithsonian Center for Astrophysics and Dartmouth College report this week.

“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,” Harvard-Smithsonian’s Avi Loeb said in a press release about the research.

The only known objects that move faster than these planets are subatomic particles, added Dartmouth’s Idan Ginsburg.

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Scientists got to thinking about runaway planets after colleagues discovered a star launching out of the Milky Way at 1.5 million miles per hour (2.4 million kilometers per hour).

Simulations showed the star — known as a hypervelocity star — must have been part of a double-star system that ended up in the wrong part of the Milky Way.

“A double-star system wanders too close to the supermassive black hole at the galactic center. Strong gravitational forces rip the stars from each other, sending one away at high speed while the other is captured into orbit around the black hole,” writes Christine Pulliam, spokeswoman for the Harvard-Smithsonian Center for Astrophysics.

Turns out the same thing can happen to planets. Ginsburg, Loeb and Darthmouth’s Gary Wegner created computer models showing what would happen if each star had a planet or two orbiting nearby.

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“They 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,” Pulliam wrote.

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

Because any tag-along planets would have to be in tight orbits around their parent stars, astronomers may be able to detect them by looking for a hypervelocity star that dims slightly as its planet passes across its face, relative to the view from Earth.

The research will be published in an upcoming edition of the Monthly Notices of the Royal Astronomical Society.

Image: Artist rendering of a runaway planet zooming  through interstellar space.  That’s a glowing volcano on the planet’s surface, a hint of  active plate tectonics to keep the planet warm. Credit: David A. Aguilar/CfA