The dynamics of an object in the 2:3 mean-motion resonance with Neptune. The double-lobed curve represents the orbit of an object with the eccentricity of Pluto. The coordinate frame rotates counterclockwise at the average speed of Neptune. Thus, Neptune (dot labeled 'N') is stationary in this figure. The location of the Sun is labeled 'S'. A) The orbit of an object whose semi-major axis is equal to that characterizing the exact location of the resonance. The gravitational perturbations of Neptune cancel out due to the symmetry in the geometry. Thus, this orbit does not precess in the rotating frame. B) If the symmetry is broken, there is a net acceleration due to Neptune. Here, the strongest perturbation (
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