The Curiously Warped Mean Plane of the Kuiper Belt

We measured the mean plane of the Kuiper belt as a function of semi-major axis. For the classical Kuiper belt as a whole (the non-resonant objects in the semi-major axis range 42–48 au), we find a mean plane of inclination im = 1.8 ◦+0.7 −0.4◦ and longitude of ascending node Ωm = 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 semi-major axis bins, we detect a statistically significant warp in the mean plane near semi-major axes 40–42 au. Linear secular theory predicts a warp near this location due to the ν18 nodal secular resonance, however the measured mean plane for the 40.3-42 au semi-major axis bin (just outside the ν18) is inclined ∼ 13◦ to the predicted plane, a nearly 3-σ discrepancy. For the more distant Kuiper belt objects of semi-major 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 im = 9.1 ◦+6.6 −3.8◦ and longitude of ascending node Ωm = 227 ◦+18 −44◦ . 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 (a . 100 au), unseen small planetary-mass object in the outer solar system is responsible for the warping.