Change in the phase angle of a planetesimal initially located at a₀ = 13.8 AU. Panel a shows the case of τ_tide,0 = 1 × 10⁴ yr and panel b shows the case of τ_tide,0 = 1 × 10⁵ yr. In panel a, the phase angle oscillates indicating that the resonant trapping is unstable due to fast planetary migration. Then, the resonant trapping is broken earlier in the case of panel a than that shown in panel b.