Pluto ’ s recently discovered minor moons , Nix and Hydra , have almost circular orbits , and are nearly coplanar with Charon , Pluto ’ s major moon .
This is surprising because tidal interactions with Pluto are too weak to damp their eccentricities .
We consider an alternative possibility : that Nix and Hydra circularize their orbits by exciting Charon ’ s eccentricity via secular interactions , and Charon in turn damps its own eccentricity by tidal interaction with Pluto .
The timescale for this process can be less than the age of the Solar System , for plausible tidal parameters and moon masses .
However , as we show numerically and analytically , the effects of the 2:1 and 3:1 resonant forcing terms between Nix and Charon complicate this picture .
In the presence of Charon ’ s tidal damping , the 2:1 term forces Nix to migrate outward and the 3:1 term changes the eccentricity damping rate , sometimes leading to eccentricity growth .
We conclude that this mechanism probably does not explain Nix and Hydra ’ s current orbits .
Instead , we suggest that they were formed in-situ with low eccentricities .

We also show that an upper limit on Nix ’ s migration speed sets a lower limit on Pluto-Charon ’ s tidal circularization timescale of > 10 ^ { 5 } yrs .
Moreover , Hydra ’ s observed proper eccentricity may be explained by the 3:2 forcing by Nix .