Randomness and Retention: Using Weak Mean Motion Resonances to Constrain Neptune's Late-Stage Migration

Abstract: Planet-planetesimal interactions cause a planet to migrate, manifesting as a random walk in semi-major axis. In models for Neptune's migration involving a gravitational upheaval, this planetesimal-driven migration is a side-effect of the dynamical friction required to damp Neptune's orbital eccentricitiy. This migration is noisy, potentially causing Trans Neptunian Objects (TNOs) in mean motion resonance to be lost. With Nbody simulations, we validate a previously-derived analytic model for resonance retention and determine unknown coefficients. We identify the impact of random-walk (noisy) migration on resonance retention for resonances up to fourth order lying between 39 au and 75 au. Using a population estimate for the weak 7:3 resonance from the well-characterized Outer Solar System Origins Survey (OSSOS), we rule out two cases: (1) a planetesimal disk distributed between 13.3 and 39.9 au with $\gtrsim$ 30 Earth masses in today's size distribution and $T_{\rm mig} \gtrsim$ 40Myr and (2) a top-heavy size distribution with $\gtrsim$ 2000 Pluto-sized TNOs and $T_{\rm mig} \gtrsim$ 10Myr, where $T_{\rm mig}$ is Neptune's migration timescale. We find that low-eccentricity TNOs in the heavily populated 5:2 resonance are easily lost due to noisy migration. Improved observations of the low-eccentricity region of the 5:2 resonance and of weak mean motion resonances with Rubin Observatory's Legacy Survey of Space and Time (LSST) will provide better population estimates, allowing for comparison with our model's retention fractions and providing strong evidence for or against Neptune's random interactions with planetesimals.