A Lopsided Outer Solar System

Abstract

Axisymmetric disks of eccentric orbits in near-Keplerian potentials are unstable to an out-of-plane buckling. Recently, Zderic et al. (2020) showed that an idealized disk saturates to a lopsided mode. Here we show that this apsidal clustering also occurs in a primordial scattered disk in the outer solar system which includes the orbit-averaged gravitational influence of the giant planets. We explain the dynamics using Lynden-Bell (1979)'s mechanism for bar formation in galaxies. We also show surface density and line of sight velocity plots at different times during the instability, highlighting the formation of concentric circles and spiral arms in velocity space.

Photometry and Kinematics of Self-Gravitating Eccentric Nuclear Disks

“…The stability of eccentric nuclear disks relies on the suppression of differential apsidal precession of disk orbits, maintained via inter-orbit gravitational torques (Madigan et al 2018). Precession also needs to be prograde with respect to the angular momentum of the disk unless the background potential is "abnormally" steep (Lynden-Bell 1979;Zderic et al 2021). If we were to include a typical spherical background stellar distribution in our simulations (due to a cusp or bulge), disk orbits would undergo an enhanced rate of apsidal precession in the retrograde direction.…”