Evolution of a test particle when transferring from symmetric to asymmetric resonance. The left column is for Neptune migrating at a rate of 1 AU Myr⁻¹, while the right column is for the same test particle but the migration stops at 0.3 Myr. From top to bottom, the panels show the semimajor axis a, eccentricity e, and resonant angle ϕ of the test particle with solid lines. The dashed lines in the top panels represent the nominal semimajor axis of the 1:2 resonance. The magenta area in the middle-left panel indicates the duration of the last symmetric cycle, while the red and blue areas in the bottom-left panel indicate the time intervals around the leading and trailing island, respectively.