Top panel: formation tracks corresponding to disc 1 (see Appendix B), Z₀ = 0.0144, and α = 10⁻⁴. The white and green circles indicate the times 0.012 Myr, 0.25 Myr, and 2 Myr. The 0.25 Myr circle of the core starting its formation just inside the ice line is below the 2 Myr circle. M_iso is reached in each simulation when M_core stops growing. The core growing in a vertical line grows so fast that it practically does not migrate before reaching M_iso. Bottom panel: evolution of the Stokes number at the planet location for the seven cases shown in the top panel. The labels indicate the initial semi-major axis. The grey circles show the time when planets enter the region r <  r_ice.