The forced orbital planes of the Plutinos and other resonant KBOs

The forced plane of non-resonant minor planets has been understood to be well-estimated by the local Laplace plane given by linear Laplace-Lagrange secular perturbation theory, but there is no extant theory for the forced plane of resonant minor planets. With improved methodology, we revisit our previous measurement of the mean orbital plane of the observed Plutinos, a group of Kuiper belt objects locked in Neptune's 3:2 mean motion resonance. In the J2000 ecliptic-equinox reference frame, the measured mean plane has an inclination of 1.9 degrees and an ascending node at 46.8 degrees longitude, with a 95% confidence of 2 degrees in the plane position; it is well separated from the local Laplace plane but is indistinguishable from the solar system's invariable plane. We also show that, at high statistical confidence, the two dynamical subgroups, the doubly resonant Plutinos whose arguments of perihelion librate about +90 and-90 degrees, have different mean planes from each other. Additionally, we measure the mean planes of four other observed resonant groups in the Kuiper belt: the 5:3, 7:4, 2:1, and 5:2. Although these groups have substantial sample sizes (in the range 56-105), their mean planes are presently not statistically distinguishable from their local Laplace plane, Neptune's plane, and the invariable plane. For all five resonant groups, we also report the best-fit von Mises function for their inclinations. These empirical measurements underscore the need for theoretical analysis to understand the spatial dynamics of resonant populations of minor planets. Such analysis could potentially enable their use for uncovering unmodeled perturbations and/or undiscovered perturbers.