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A cluster of icy bodies in the same region as Pluto could be proof that our early solar system was home to a fifth giant planet, according to new research. That planet may have “bumped” Neptune during its migration away from the sun 4 billion years ago, causing the ice giant to jump into its current orbit and scattering a cluster of its satellites into the Kuiper belt in the outer solar system.
The cluster—a grouping of about a thousand icy rocks called the “kernel”—has long been a mystery to astronomers. The rocks stick close together and never veer from the same orbital plane as the planets, unlike the other icy bodies that inhabit the belt. Previous studies proposed that the tightly bound objects formed from violent collisions of larger parent bodies, but that hypothesis fell apart as soon as scientists realized these collisional families would have to be stretched across the Kuiper belt.
But now, one scientist may have an answer for this Kuiper belt mystery. David Nesvorny, an astronomer at the Southwest Research Institute in Boulder, Colorado, proposes the jumping Neptune theory in the September issue of The Astronomical Journal. Using computer simulations to trace the movements of the kernel back about 4 billion years, he found the objects had been swept up in Neptune’s gravitational field as the planet migrated away from the sun. Leaving its orbit near Saturn and Jupiter, Neptune pulled bits of the primordial solar system along with it as they rotated in tandem: The infant kernel traveled around the sun twice for every trip that Neptune made.
At approximately 4.2 billion kilometers from the sun—close to its current position almost halfway to the outer edge of the modern-day Kuiper belt—Neptune’s orbit lurched outward 7.5 million kilometers. The trapped objects couldn't keep up with the sudden change of pace, and they were jolted out of their orbital configuration 6.9 billion kilometers from the sun, where they continue to travel today as the kernel.
Nesvorny says the only possible explanation for the sudden shift in orbit is that Neptune came under the gravitational sway of another object with a massive gravitational field—likely a giant planet. Uranus, Saturn, and Jupiter aren’t candidates because their orbits have never interacted with Neptune’s in the way that this proposed planet’s might have done.