Resonance trapping of the HR 8799 system during the migration phase. The upper panel shows the evolution of the semi-major axis of the outer planet at the moment of capture in resonance with the inner three bodies. After the resonance trapping, the semi-major axis remains constant. The middle panel shows the critical argument of the resonance between the two outer planets. After the capture in resonance, the critical argument librates. The same occurs for the Laplace critical argument shown in the bottom panel.