The destabilization of some of Neptune’s distant mean motion resonances by Uranus

Many of Neptune’s external mean motion resonances (MMRs) are occupied by observed transneptunian objects (TNOs), though the detailed dynamics of the most distant MMRs remains under-explored. Our prior work mapping Neptune’s external MMRs in simulations of the restricted three-body problem (Sun + Neptune + massless test particles) show that they are strong and dynamically significant out to at least 250-500 au (depending on perihelion distance). However, when the other giant planets are added to our simulations, there are notable changes to the extent and strength of the MMRs, especially for particles with perihelion distances consistent with the scattering TNO population (q < 37-38 au). We applied a Poincare mapping approach, recently expanded to the multi-planet case in Volk & Malhotra (2022), to measure the widths of Neptune’s MMRs in simulations with all four giant planets. We mapped Neptune’s 10:1 through 30:1 MMRs at q=33 au and noted a sudden drop in N:1 resonant strengths at the 20:1 (~220 au) that continues through the 25:1 (~260 au). We simulated the 20:1 at q=33 au with different combinations of giant planets to isolate the underlying destabilization mechanism. Jupiter and Saturn did not strongly affect the resonance, but simulations with Uranus present destabilized the 20:1. We ran a simulation including Jupiter, Saturn, Neptune, and a J2 parameter to induce Uranus’s secular effects; the 20:1 is stable in this simulation. We simulated the same range of N:1 MMRs with all the giant planets for larger perihelion distances and determined that the destabilization of the 20:1 occurs only at low perihelia; the 20:1 remains dynamically strong for q=40-45 au. We explored whether the overlap of Neptune’s 2-body MMRs with Uranus’s 2-body MMRs could explain the destabilization at higher-eccentricities, but maps of both planets’ MMRs in the corresponding simplified problems do not support this as the underlying cause. We speculate that higher-order 3-body resonances (between Neptune, Uranus, and the test particles) might drive the disappearance of some distant MMRs at low perihelion distances. The destabilization of Neptune’s MMRs for high eccentricity TNOs from 220-260 au is caused by Uranus, and the position of Uranus is pertinent to the destabilization. This location for the drop off of MMRs at low perihelia is consistent with simulations of the scattering population of TNOs, which show drop-offs in resonant ‘sticking’ at ~250 au.

This work is supported by NASA grant 80NSSC19K0785.