Change of semi-major axis as a function of time for a planet with a constant mass of 20 Earth masses embedded in our disc model at initially 10 AU for different prescriptions of α (top) and for different chemical compositions for the opacity (bottom). All simulations in the bottom plot feature α = 0.001 and u_f = 1 m s⁻¹. The two opacity descriptions using Bitsch et al. (2015a) and Emsenhuber et al. (2020) for the envelope opacity use pure micrometer grain opacities for migration, resulting in identical migration patterns.