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 kilometres. UWBs are dynamically fragile, and their existence is thought to constrain the early Solar System processes and conditions. However, we demonstrate that UWBs can instead attain their wide architectures well after the earliest epochs of the Solar System, when the orbital migration of Neptune 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.

占据最圆、共面柯伊伯带轨道（冷经典带）的天体中，近三分之一是双星，其中百分之几是“超宽”双星 （UWB）：~100 公里大小的伴星，间隔数万公里。UWB 在动态上是脆弱的，它们的存在被认为限制了太阳系早期的过程和条件。然而，我们证明，UWB 可以在太阳系最早的时代之后很久就获得其宽架构，当时海王星的轨道迁移植入了现代非寒冷或“动态”柯伊伯带种群。在这种植入过程中，冷经典带双星可能会与散布在该区域的许多行星体近距离相遇，这可以有效地解离任何现有的 UWB，并将一小部分更紧密的双星扩大为类似 UWB 的排列。因此，今天的 UWB 可能不是原始的，不能像以前推测的那样直接用于限制早期太阳系。