More Realistic Planetesimal Masses Alter Kuiper Belt Formation Models and Add Stochasticity

Abstract

We perform simulations here that include the gravitational effects of the primordial planetesimal belt consisting of ~10^5 massive bodies. In our simulations, Neptune unlocks from resonance with the other giant planets and begins to migrate outward due to interactions with planetesimals before a planetary orbital instability is triggered, and afterward, residual Neptunian migration completes the formation of the modern Kuiper belt. Our present work exhibits a number of notable differences from prior work. First, Neptune's planetary resonance unlocking requires the Neptunian 3:2 mean motion resonance to sweep much of the primordial disk interior to 30 au prior to the giant planet instability. The pre-instability population of planetesimals is consequently lower in semimajor axis, eccentricity, and inclination, and this effect persists after the instability. Second, direct scattering between Pluto-mass bodies and other small bodies removes material from Neptunian resonances more efficiently than resonant dropout resulting from small changes in Neptune's semimajor axis during scattering between Pluto-mass bodies and Neptune. Thus, the primordial population of Pluto-mass bodies may be as few as ~200 objects. Finally, our simulation end states display a wide variety of orbital distributions, and clear relationships between final bulk Kuiper belt properties and Neptune's migration or initial planetesimal properties largely elude us. In particular, we find that the rapid, stochastic planetary orbital evolution occurring during the giant planet instability can significantly alter final Kuiper belt properties such as its inclination dispersion and the prominence of resonant populations. This complicates using modern Kuiper belt properties to confidently constrain early solar system events and conditions, including planetary orbital migration and the primordial Kuiper belt's characteristics.