What this model simplifies

• 2D. Bodies move in a plane, but gravity falls off as 1/r² (a thin 3D disk, not "flatland" gravity). With no vertical dimension, close encounters end in contact more often than in 3D.
• Pinned star by default. The star is a fixed point that pulls but is never pulled back. That drops the "indirect term" (the star's reflex motion toward its planets). It hardly matters for close encounters, but it removes the Lagrange points L4/L5 entirely: Trojans exist only with the Live star option, which the Trojan, Routh, chaotic-zone and resonance presets use.
• No gas. Real disks are mostly gas at first. Without it there are no gas giants, no gas drag damping orbits and no planet migration, and the asteroid belt is not stirred by a forming Jupiter.
• Perfect merging. Bodies that touch always stick and conserve mass and momentum; nothing shatters or bounces.
• Radius inflation. In the disk presets bodies collide as if ~30× larger than their density implies, so growth takes minutes instead of millions of years. That changes how often close passes merge rather than scatter (FINDINGS §3 and §6).
• Two integrators. Leapfrog (default, fast) and a hybrid symplectic one (exact Kepler orbits plus resolved close encounters, for science runs).

Each preset checks the simulation against a known answer. The full list, with tolerances and measured values, is in VALIDATION.md; the science results are in FINDINGS.md.