The orbits of icy minor planets beyond Neptune are doing something very strange: they all tilt and pitch in similar ways, and maybe even cluster together on the sky. In a 2016 paper, my collaborator Mike McCourt and I show that when gravitational forces between minor planets on eccentric orbits are included in N-body simulations, the orbits incline rapidly off the disk plane, and tilt and pitch in exactly the same way. This gravitational instability is something that a disk of eccentric orbits does on its own -- we don't need to invoke an external reason (like a planet 9) for it. We do, however, predict a lot of minor planets in the outer Solar System, a "new Kuiper Belt" (with an order of magnitude more mass!), to defend the orbital clustering against the precessing effects of the giant planets.
We now have a whole series of papers on this topic, including an explanation of the dynamics behind the collective gravity mechanism and the discovery of eccentricity oscillations when we include a spectrum of minor planet masses.
We recently (April 2020) submitted a paper on the effects of the giant planets on a primordial scattered disk and a letter showing that the collective gravity of extreme Trans-Neptunian Objects can result in apsidal clustering.
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