The wide-binary origin of the Pluto-Charon system

The Pluto-Charon binary system is the best studied representative of the binary Kuiper-belt population. Its origins are vital to understanding the formation of other Kuiper-belt objects (KBO) and binaries and the evolution of the outer Solar system. The Pluto-Charon system is believed to form following a giant impact between two massive KBOs at relatively low velocities. However, the likelihood of a random direct collision between two of the most massive KBOs is low and is further constrained by the requirement of a low-velocity collision, making this a potentially fine-tuned scenario. Here, we expand our previous studies and suggest that the proto-Pluto-Charon system was formed as a highly inclined wide-binary, which was then driven through secular/quasi-secular evolution into a direct impact. Since wide-binaries are ubiquitous in the Kuiper belt with many expected to be highly inclined, our scenario is expected to be robust. We use analytic tools and few-body simulations of the triple Sun-(proto-)Pluto-Charon system to show that a large parameter space of initial conditions leads to such collisions. The velocity of such an impact is the escape velocity of a bound system, which naturally explains the low-velocity impact. The dynamical evolution and the origins of the Pluto-Charon system could therefore be traced to similar secular origins as those of other binaries and contact-binaries (e.g. Arrokoth) and suggest that they play a key role in the evolution of KBOs.