Number of particles in the leading and trailing side of the horseshoe orbits during the nominal model migration and growth of Neptune. The blue line is the number of particles in the L₄ side of the horseshoe orbit (leading side) versus time and the green line is the number of particles in the L₅ side of the horseshoe orbit (trailing side) versus time. We can clearly see an excess of leading particles during the growth and migration due to the deformations of the horseshoe orbit, but Neptune has not grown enough to shrink the orbits to stable tadpole orbits in order to preserve the asymmetry. Then when the migration stops the excess simply disappears because the horseshoe orbits are back symmetric. Dashed lines indicate the time frame where Neptune is growing and migrating.