A Non-Primordial Origin for the Widest Binaries in the Kuiper Belt

Nearly one-third of objects occupying the most circular, coplanar Kuiper belt orbits (the cold classical belt) are binary companions revolving about one another as they orbit the Sun, and several percent of binaries are “ultra-wide” binaries (UWBs): ∼100-km companions spaced by tens of thousands of km. UWBs are dynamically fragile, and their existence is thought to constrain early solar system processes and conditions. However, we demonstrate that UWBs can attain their wide architectures well after the solar system’s earliest epochs when Neptune’s orbital migration implants the modern non-cold, or “kinetic,” Kuiper belt. During implantation, huge numbers of planetesimals cross the in-situ cold classical belt, and cold classical belt binaries suffer close encounters with them. Such encounters efficiently dissociate any existing UWBs and widen a small fraction of tighter binaries into UWB-like arrangements. Thus, today’s UWBs may not be primordial and may constrain the early solar system differently than previously surmised.