SIMBAD references

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 i_m=1○ / . 8_-0○ / . 4_^+0○ / . 7^ and longitude of ascending node Ω_m=77^○-14_○+18○^ (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 ν₁₈ nodal secular resonance; however, the measured mean plane for the 40.3-42 au semimajor axis bin (just outside the ν₁₈) is inclined ∼13^○ to the predicted plane, a nearly 3σ discrepancy. For the more distant Kuiper Belt objects of semimajor axes in the range 50-80 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 i_m=9○ / . 1_-3○ / . 8_^+6○ / . 6^ and longitude of ascending node Ω_m=227^○-44_○+18○^. 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.