Distribution of test particles captured by the sweeping 1:2 resonance. The resonance swept through these particles as Neptune migrated at a constant rate of 5 AU Myr⁻¹, and the migration was stopped when the nominal resonance reached 47.62 AU. The abscissa and ordinate are for the initial semimajor axis and eccentricity of test particles. The red and blue colours indicate that an object was captured by the leading and trailing resonance island, respectively. The dashed line at 47.35 AU separates test particles into two groups (see text).