Current theory considers two options for the formation of the Pluto–Charon binary ( e.g. , ) .
In the ‘ hit-and-run ’ model , a lower mass projectile barely hits the more massive Pluto , kicks up some debris , and remains bound to Pluto ( see also ) .
In a ‘ graze-and-merge ’ scenario , the projectile ejects substantial debris as it merges with Pluto ( see also ) .
To investigate the graze-and-merge idea in more detail , we consider the growth of Charon-mass objects within a circum-Pluto ring of solids .
Numerical calculations demonstrate that Charon analogs form rapidly within a swarm of planetesimals with initial radii r _ { 0 } \approx 145–230 km .
On time scales of \sim 30–100 days , newly-formed Charon analogs have semimajor axes , a \approx 5–6 r _ { P } , and orbital eccentricities , e \approx 0.1–0.3 , similar to Charon analogs that remain bound after hit-and-run collisions with Pluto .
Although the early growth of Charon analogs generates rings of small particles at a \approx 50–275 r _ { P } , ejection of several 145–230 km leftovers by the central Pluto–Charon binary removes these small solids in 10–100 yr .
Simple estimates suggest small particles might survive the passage of 10–20 km objects ejected by the central binary .
Our results indicate that the Pluto–Charon circumbinary satellite system was not formed by a graze-and-merge impact when the formation of Charon within a circum-Pluto disk leads to the ejection of several 100–200 km particles through the orbital plane of the Pluto–Charon binary .
If a growing Charon ejects only much smaller particles , however , graze-and-merge impacts are a plausible formation channel for the Pluto–Charon binary and an ensemble of small , circumbinary satellites .