About 300 light-years away, a giant planet has been evicted from its starry neighborhood. The world is now about 650 times farther from its star than Earth is from the sun, scientists announced Tuesday at the Extreme Solar Systems III meeting in Hawaii.
That’s more than 16 times farther from the sun than Pluto is.
But the planet, known as HD 106906b, didn’t end up in the suburbs by chance: Scientists suspect giant gravitational nudges from another planet—or perhaps from a passing star—sent it zooming outward. It’s the same kind of process that scientists blame for the billions of rogue, starless worlds that wander our galaxy. They’ve just never really seen it in action.
“This whole picture of a dynamically disturbed planetary system is tremendously exciting,” says the SETI Institute’s Paul Kalas, who was part of the team that observed the planet using the Gemini Planet Imager in the Chilean Andes.
The planet is about 11 times more massive than Jupiter and orbits a star a bit bigger than the sun. Yet at only 13 million years old, the system is much younger than our own. Nearer the star is a stirred-up disk of comets that astronomers think is like a larger, more chaotic version of our solar system’s Kuiper Belt, the icy region beyond the orbit of Neptune where Pluto and countless other worlds reside.
“We think the whole system has been recently disturbed by some violent gravitational interaction,” says Kalas, who is also affiliated with the University of California, Berkeley.
In fact, observations suggest it’s possible that as the planet got punted from the system, it took some of the debris from that icy comet disk for a ride, says Abhijith Rajan, a graduate student at Arizona State University. It’s dustier than expected, and could be surrounded by a large ring or debris cloud.
Normally, the planetary bodies orbiting a young star sit in the same relatively flat plane, kind of like the grooves on a vinyl record; that means HD 106906b should be in the same plane as the exo-Kuiper Belt. But in addition to being on the extreme fringe, the planet hovers far, far above the dusty belt. That means it either formed way out there (which astronomers think is unlikely) or that some kind of gravitational kerfuffle tossed it from its early neighborhood.
“Planets don’t form outside the plane of the planetary system,” Kalas says simply.
Scientists suspect they’re getting a glimpse of the growing pains that young planetary systems go through, when planets push and pull on one another and sometimes erupt in violence.
Our own planetary system experienced this type of spasm around four billion years ago, when the giant planets began migrating outward. In the resulting chaos, small bodies went flying, Uranus and Neptune may have switched places, and there’s a good chance that gravitational nudges from Jupiter ejected a fifth giant planet, which is now lost in space.
Whether HD106906b keeps migrating outward or has settled into a spot that’s just absurdly far from its star is yet to be seen. Rajan suggests it might stay put, but there’s a chance the planet could become one of the many untethered worlds that roam the galaxy without home stars—planets adrift in a sea of perpetual night.
“It is entirely possible that through chaotic evolution, this planet will eventually find itself going rogue,” says Caltech’s Konstantin Batygin.
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