The Curiously Warped Mean Plane of the Kuiper Belt

Abstract

Abstract We measured the mean plane of the Kuiper Belt as a function of semimajor axis. For the classical Kuiper Belt as a whole (the nonresonant objects in the semimajor axis range 42–48 au), we find a mean plane of inclination and longitude of ascending node (in the J2000 ecliptic-equinox coordinate system), in accord with theoretical expectations of the secular effects of the known planets. With finer semimajor axis bins, we detect a statistically significant warp in the mean plane near semimajor axes 40–42 au. Linear secular theory predicts a warp near this location due to the nodal secular resonance; however, the measured mean plane for the 40.3–42 au semimajor axis bin (just outside the ) is inclined to the predicted plane, a nearly 3σ discrepancy. For the more distant Kuiper Belt objects of semimajor axes in the range 50–80 au, the expected mean plane is close to the invariable plane of the solar system, but the measured mean plane deviates greatly from this: it has inclination and longitude of ascending node . We estimate this deviation from the expected mean plane to be statistically significant at the ∼97%–99% confidence level. We discuss several possible explanations for this deviation, including the possibility that a relatively close-in ( au), unseen, small planetary-mass object in the outer solar system is responsible for the warping.