The curiously warped mean plane of the kuiper belt

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-V48 au), we find a mean plane of inclination i_m 1°.8^+0°7 _-0°4 and longitude of ascending node Ω_m = 77°^+18° _-14° (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-V42 au. Linear secular theory predicts a warp near this location due to the v₁₈ nodal secular resonance; however, the measured mean plane for the 40.3-V42 au semimajor axis bin (just outside the v₁₈) is inclined ~13° to the predicted plane, a nearly 3σ discrepancy. For the more distant Kuiper Belt objects of semimajor axes in the range 50-V80 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 i_m= 9°.1^+6°6 _-3°8 and longitude of ascending node Ω_m= 227°^+18° _-44°. We estimate this deviation from the expected mean plane to be statistically significant at the ~97%-V99% confidence level. We discuss several possible explanations for this deviation, including the possibility that a relatively close-in (a ≲ 100 au), unseen, small planetary-mass object in the outer solar system is responsible for the warping.