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

TL;DR

This paper proposes that ultra-wide Kuiper belt binaries are likely formed after the Solar System's formation through Neptune's migration, challenging previous assumptions that they are primordial and constraining early Solar System conditions.

Contribution

It introduces a new formation mechanism for ultra-wide Kuiper belt binaries involving Neptune's migration and planetesimal encounters, altering their use as early Solar System probes.

Findings

UWBs can form after Neptune's migration through binary dissociation and widening.

Most UWBs are not primordial but formed during later dynamical processes.

The existence of UWBs does not necessarily constrain early Solar System conditions.

Abstract

Nearly one-third of objects occupying the most circular, coplanar Kuiper belt orbits (the cold classical belt) are binary, and several percent of them are "ultra-wide" binaries (UWBs): 100-km-sized 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 instead attain their wide architectures well after the Solar System's earliest epochs, when Neptune's orbital migration implants the modern non-cold, or "dynamic", Kuiper belt population. During this implantation, cold classical belt binaries are likely to have close encounters with many planetesimals scattered across the region, which can 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 cannot be used to constrain the early Solar System as directly as previously surmised.