All the hubbub over Pluto’s status in the solar system has made it tricky to keep track of how many planets we have. Now there’s another factor to make things even more confusing: scientists believe that our system once had another planet between Neptune and Pluto.
David Nesvorny, an astronomer at the Southwest Research Institute in Boulder, Colorado, proposes a theory that may solve the mystery of the Kuiper Belt Kernel. The ‘kernel’ refers to a cluster of icy rocks that floats together within the same orbital plane as the planets. Their close association distinguishes them from the rest of the Kuiper Belt, a region of small bodies that extends from Neptune’s orbit to the edge of the solar system. Astronomers used to think the kernel is comprised of debris left over from violent collisions of larger bodies, but that theory doesn’t explain why they stick so closely together.
Nesvorny’s jumping Neptune theory could explain the behavior of the kernel. He used computer simulations and models to extrapolate the kernel’s movements back through the solar system’s history, and discovered that the icy fragments were pulled into Neptune’s gravitational field when the planet first moved away from the Sun.
But then Neptune made a sudden shift in its course, leaping outwards about 7.5 million kilometers. There, the rocks were knocked out of Neptune’s pull, but they continue to drift in close affiliation today.
But what caused Neptune to jump off course? Nesvorny believes that only an equally massive object—another planet, for instance—could have exerted a strong enough gravitational pull. Uranus, Saturn, and Jupiter are innocent, as they have never interacted with Neptune’s orbit in a way that would have caused this jump.
In addition to resolving the origins of the Kuiper Kernel, a fifth giant planet would explain the solar system’s present day orbital configuration. Previous theories of the solar system’s formation that only account for four giant planets don’t make sense in terms of their current locations.
The jumping Neptune theory helps us keep in mind that the solar system as we know it is the result of intense, violent battles involving burgeoning planets and their opposing gravitational pulls. This absentee planet was most likely ejected as the survivors settled down into stable configurations, and the Kuiper Belt Kernel remains as the only trace of its existence.
Scientists continue to analyze the various rocky fragments strewn between the planets to gain a better understanding of our solar system’s history. The Outer Solar System Origins Survey, a project that uses telescope data to determine orbits, will provide additional information.