whatA cold disk of thousands of planetesimals around a star. Two rules beyond gravity: bodies that touch MERGE (momentum kept, radius growing as the cube root), and -- the standard oligarchic approximation, declared -- small bodies feel only the star and the grown; only embryos past a threshold pull on the rest. Collision radii are drawn, and collide, a thousand times fat, or nothing would meet inside a minute.

whyThe claim: oligarchic growth. Accretion is not fair -- whoever gets slightly ahead has more cross-section and more gravity, gets ahead faster, and runs away with its neighborhood. A few embryos emerge, sweep their feeding lanes clean (watch the gaps open in the semi-major-axis histogram), and, competing for the same food, space themselves at a handful of mutual Hill radii -- the same spacing the solar system's planets keep today. The HUD measures the spacing live.

howLeapfrog around the star with embryo gravity only (softened); merging via a spatial hash every fourth step; a gentle declared cooling stands in for dynamical friction. The slider bends time. The oligarch spacing is measured between neighbours in units of their mutual Hill radius. One integer seeds the disk.