Phantom Planet Posited as Fifth Outer Giant

An amalgam of icy bodies occupying roughly the same region of space as Pluto might be proof that the early solar system used to be home to a fifth giant planet, scientists have hypothesized. If it exists, this mystery giant’s gravity might have had a brief but major interaction with Neptune’s gravity well while Neptune was already on its migration away from the sun 4 billion years ago. This would have caused the ice giant to jump to its current, more distant orbit, and scattered a cluster of its satellites into the Kuiper belt, and the far reaches of our own solar system.


This cluster of thousands of icy rocks, lovingly known by the name “kernel,” has been a source of mystery for astronomers for quite some time. The rocks composing the kernel huddle together, and never stray from the orbital plane of the planets, much different from other bodies of ice present in the belt. Studies performed before have hypothesized that such kernels of ice are the result of violent collisions of bigger parent bodies of ice, but this proposal could not stand up to scrutiny after scientists learned that these descendants of collision should then be scattered across the entire Kuiper belt.


However, recently one scientist in particular believes he’s discovered an empirical narrative that happens to fit the facts of this Kuiper belt mystery. An astronomer named David Nesvorny at the Southwest Research Institute in Boulder, Colorado, thinks the jumping Neptune theory (published in the September issue of The Astronomical Journal) has merit. Tracing the kernel’s movements back 4 billion years with the help of computer simulations, David discovered that such icy objects were swept up in Neptune’s gravitational field as it moved farther from the sun, long, long ago. Originally (if such an adjective makes sense on a solar scale), the blue gas giant orbited near Saturn and Jupiter, but as it left their vicinity, Neptune tugged pieces of the early star system with it, and they began to follow a revolve around the sun at about twice the pace of Neptune.


Roughly 4.2 billion kilometers (2.61 billion miles) from the sun, having nearly reached its current home about halfway to the Kuiper belt’s present edges, Neptune’s orbit was jerked out of normal sync, and migrated out an additional 7.5 million kilometers (4.66 million miles). This happened faster than the cluster of icy bodies could cop, so they were suddenly launched out of their orbit to one 6.9 billion kilometers (4.29 billion miles) from the sun, where they remain to the present day.

Nesvorny believes the only possible way such a sudden alteration in the kernel’s orbit could have occurred is if Neptune experienced a gravitational distortion from another body, of comparable gravitational force. It can’t have been the work of Uranus, Saturn or Jupiter, since their orbits have never interacted with Neptune’s in a way specific enough to create this cluster’s behavior. It thus must be David’s giant phantom planet.

Scientists are unsure what might have happened to the phantom planet, but Nesvorny’s earlier planetary model from 2011 could only account for the present’s orbital patterns by positing a fifth giant planet.


It’s likely that the phantom planet was ejected from the solar system after changing the orbits of the surviving planets, said Nesvorny. Not all planets formed in an early solar system survive the process of settling orbits.

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


Before choosing his current model of Kernel formation, Nesvorny tested 100 other alternative models. JJ Kavelaars of the Dominion Astrophysical Observatory in Victoria, Canada levels that one of the most difficult parts of modeling is to successfully predict the motion of multiple objects in the solar system so that they still end up in their present locations, with the same velocities, with the same trajectories.

Nesvorny’s next step is to identify more Kuiper belt objects, specifically those still within the kernel. This will in turn assist future scientists as they endeavor to improve model accuracy, and David believes studying the origins of the outer solar system at summer’s end is the way to go.