Kuiper Belt Formation via Grainy Planetary Migration

Abstract

We used N-body simulations to model the 4.5 Gyr orbital evolution of the early Kuiper Belt, incorporating a massive protoplanetary disk, the four giant planets, and 1500 primordial Pluto-class bodies ("Plutos") that drove Neptune's grainy migration. The analysis of 67 simulated systems revealed key insights: (1) All systems featured the primary trans-Neptunian object (TNO) populations: cold/hot classical, resonant, scattered, and detached; (2) Captures into stable resonant orbits favored close Neptunian mean motion resonances (MMRs; e.g., 3:2, 2:1), while distant ones beyond 50 au (e.g., 5:2 MMR) were underpopulated; (3) Optimal matches to observed resonant fractions and the classical region (including the kernel) arose from models considering a jumping Neptune, self-gravitating Plutos, and an initial disk edge at 45-47 au; (4) Models including primordial scattered disks boosted distant MMR captures but overproduced scattered objects; (5) All models were inefficient at producing the detached (q > 40 au) and high-i (i > 45 deg) populations and failed to populate observed niches, such as distant detached (a > 245 au), low-i detached (i < 20 deg), low-i scattered with q = 37-40 au (i < 20 deg), and extreme (q > 50 au or i > 50 deg) TNOs; (6) Grainy migration effects peaked early, fading as the Plutos were removed; (7) With a few primordial Plutos surviving inside 50 au, the initial population was estimated at ~150-500 to explain Pluto's solitary status. Although our four-giant-planet models reasonably replicate the trans-Neptunian structure within 50 au, they fail to account for detached, high-i, and extreme TNOs. Additional processes (e.g., a distant undiscovered planet) are required for a comprehensive outer solar system framework.