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 disc, 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 favoured close Neptunian mean motion resonances (MMRs; e.g. 3:2 and 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 disc edge at 45–47 au; (4) models including primordial scattered discs 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°) populations and failed to populate observed niches, such as distant detached (a > 245 au), low-i detached (i < 20°), low-i scattered with q = 37–40 au (i < 20°), and extreme (q > 50 au or i > 50°) TNOs; (6) grainy migration effects peaked early, fading as the Plutos were removed; and (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.