Evolution of the solid (panel a), gas (panel b), and total torques (panel c) exerted on a 1 M_⊕ planet over time (measured in terms of the number of orbits). The torques are normalized by the factor Γ₀ = 2/q. The different models – with and without the back-reaction of the solid material – are represented by the empty and filled circles, respectively. The colors indicate the different types of solid material.