Researchers have identified a compact cluster of objects, dubbed the inner kernel, within the solar system's Kuiper belt. This discovery, made by analyzing orbits of thousands of Kuiper belt objects, reveals an ancient, undisturbed formation at about 43 astronomical units from the sun. The finding offers insights into the early solar system's evolution, including Neptune's migration.
The Kuiper belt, a disc of icy rocks on the outermost edges of the solar system, has revealed more complexity than previously understood. In 2011, scientists discovered a cluster of objects on similar orbits, named the kernel, located about 44 astronomical units (AU) from the sun, where one AU is the Earth-sun distance.
A team led by Amir Siraj at Princeton University in New Jersey refined orbital data from 1650 Kuiper belt objects (KBOs) and applied an algorithm to detect clustering. The algorithm consistently identified the original kernel alongside a more compact group, which the researchers called the inner kernel due to its position at about 43 AU. All objects in this inner kernel exhibit remarkably circular orbits aligned with the solar system's disc.
"That kind of orbital calmness is a signal of a very old, undisturbed structure – the kind of structure that can provide clues to the evolution of the solar system, how the giant planets have moved in their orbits, what kind of interstellar environments the solar system has been through, all sorts of things about the early days of the solar system," says Siraj.
This structure may shed light on Neptune's migration from the inner solar system to its current position. David Nesvorný at the Southwest Research Institute in Colorado, who co-discovered the original kernel, suggests that as Neptune moved outward, it could have temporarily captured these KBOs through gravitational interactions, leading to the observed clumping before releasing them.
The Vera C. Rubin Observatory in Chile, which began operations this year, is expected to discover many more KBOs, potentially revealing additional structures. "The more we learn about the architecture of the Kuiper belt, the more we learn about the solar system’s history," says Siraj.
The findings are detailed in a preprint on arXiv (DOI: 10.48550/arXiv.2511.07512).