Regarding the Putative Eccentricity of Charon’s Orbit

Based on astrometry from an orbit derived byHubble Space Telescope imagery, Charon’s orbital eccentricity has been reported to be in the range of 0.003–0.008. Solar and planetary tides are orders of magnitude too small to induce the reported eccentricity. This nonzero value, if correct, therefore indicates some significant forcing against the two-body tidal equilibrium value, which should formally be zero. Here we follow up on a preliminary study to investigate whether the reported eccentricity of Charon’s orbit could be due to gravitational perturbations by Kuiper belt object (KBO) flybys through the Pluto-Charon system and KBO impacts directly onto Pluto and Charon. We find it is unlikely that Charon’s reported eccentricity could be caused by this effect. Although we cannot rule out some additional source of eccentricity excitation (e.g., an undiscovered satellite in the system, or a Kozai resonance), our analysis indicates it is plausible that Charon’s actual orbital eccentricity is substantially smaller than the 0.003 lower limit reported previously.