Suppression of the inclination instability in the trans-Neptunian Solar system

Abstract: The trans-Neptunian scattered disk exhibits unexpected dynamical structure, ranging from an extended dispersion of perihelion distance to a clustered distribution in orbital angles. Self-gravitational modulation of the scattered disk has been suggested in the literature as an alternative mechanism to Planet 9 for sculpting the orbital architecture of the trans-Neptunian region. The numerics of this hypothesis have hitherto been limited to $N < O(10^3)$ super-particle simulations that omit direct gravitational perturbations from the giant planets and instead model them as an orbit-averaged (quadrupolar) potential, through an enhanced $J_2$ moment of the central body. For sufficiently massive disks, such simulations reveal the onset of collective dynamical behaviour $\unicode{x2014}$ termed the $\unicode{x2018}$inclination instability$\unicode{x2019}$ $\unicode{x2014}$ wherein orbital circularisation occurs at the expense of coherent excitation of the inclination. Here, we report $N = O(10^4)$ GPU-accelerated simulations of a self-gravitating scattered disk (across a range of disk masses spanning 5 to 40 Earth masses) that self-consistently account for intra-particle interactions as well as Neptune's perturbations. Our numerical experiments show that even under the most favourable conditions, the inclination instability never ensues. Instead, due to scattering, the disk depletes. While our calculations show that a transient lopsided structure can emerge within the first few hundreds of Myr, the terminal outcomes of these calculations systematically reveal a scattered disk that is free of any orbital clustering. We conclude thus that the inclination instability mechanism is an inadequate explanation of the observed architecture of the solar system.