Our early solar system may have been home to a fifth giant planet

Primordial particles filled the outer solar system early in its lifetime. As the orbits of Jupiter (green circle), Saturn (orange circle), Neptune (dark blue circle), and Uranus (light blue circle) shifted over time, gravitational interactions tossed many of these icy rocks into the region today known as the Kuiper belt.

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.

No one knows what became of the missing planet, but when Nesvorny developed a previous model in 2011, he found that the best way to wind up with the present-day orbital configuration of the solar system was to include a fifth giant planet. The mystery giant was most likely ejected permanently from the solar system after disrupting the original orbits of the surviving planets, Nesvorny says—a casualty of its gravitational wrestling match. But back in 2011, he never thought he would find evidence of the planet’s possible existence.

“The Kuiper belt is the clue,” Nesvorny says. “You see the structures there, and you try to figure out what kind of evolution would fit those structures.”

But trying to figure it out isn’t so simple. Before landing on his current model of how the kernel could have formed, Nesvorny tested about 100 other possibilities. One of the most difficult parts of modeling, says astronomer JJ Kavelaars of the Dominion Astrophysical Observatory in Victoria, Canada, is to get multiple objects in the solar system to end up in the right place. Early models for the formation of Pluto, for example, put the dwarf planet in the correct location but neglected other parts of the solar system.

"What Nesvorny's models are doing is being very self-consistent and getting multiple structures right at once, which is really quite amazing," says Kavelaars, who was not involved in the research.

Nesvorny says the next step is to identify more objects in the Kuiper belt, particularly in the kernel. This could help scientists perform more precise comparisons that could improve the accuracy of the model. Nesvorny hopes observations from the Outer Solar System Origins Survey later this summer will fill in some of the gaps.

"I'm very much looking forward to seeing what kind of observations they will have and how it fits the modeling," Nesvorny says.