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tokens: was larger than the current discovered value, increasing the critical migration rate. The rate of migration can slow as the planets approach the inner edge of the disc due to the decrease in surface density (Kley Haghighipour 2014 , 2015 ). This slowing makes the likelihood of scattering more probable since it increases the time of interaction with the N:1 resonances. Additionally, if there exists a lack of circumbinary planets and especially multiplanet systems, this might be indicative of slow migration rates. 6.3 Application to the Pluto and Charon binary With a mass ratio of 0.1184, the dynamics around Pluto and its largest moon, Charon, behaves similar to the binary star systems described above. The binary dwarf planet has four discovered circumbinary moons: Styx, Nix, Kerberos, and Hydra. These moons are near to the 3:1, 4:1, 5:1, and 6:1 resonances with Charon and are therefore close to resonances with each other. None of these resonances are currently active according to the best estimates for the orbits and masses of the moons (Showalter Hamilton 2015 ). The very low eccentricities of all the bodies in the system makes active resonances unlikely due to the small resonance widths. Showalter Hamilton ( 2015 ) suggests the masses of the circumbinary moons are larger than predicted by albedo alone (Brozović et al. 2015 ) due to a librating three-body resonant angle for higher masses. While moon–moon resonances may help constrain the mass of the moons, the location of the moons near the N:1 resonances remains unexplained. The very low eccentricity of the moons suggests that in situ formation is likely (Brozović et al. 2015 ) but the collision from which the binary formed is not thought to have created enough leftover material at large enough distance to form the moons at their current location (Canup 2005 , 2011 ). Bromley Kenyon ( 2015 ) demonstrate that processes like physical collisions and collective gravitational scattering can expand the debris in the system to larger distances, facilitating formation. Resonant transport cannot explain the migration of all moons simultaneously since different N:1 resonances require different binary eccentricity for migration (Lithwick Wu 2008 ; Cheng et al. 2014 ). Resonant transport due to an eccentric Charon would also excite the eccentricity of the moons. Even outside of a resonance, the secular effects can excite the eccentricity of the moons above their current values. Formation models that predict a high initial eccentricity for the binary need to explain the current low eccentricity of the moons and overcome the likelihood of instabilities from the number of resonances in an eccentric system. Previous efforts have used stability criteria to constrain the maximum eccentricity of Charon as well as the maximum mass of the moons (Youdin, Kratter Kenyon 2012 ; Smullen Kratter 2017 ). The instabilities described here can also limit the possible masses and formation history of the moons. The potential