Gas accretion rates on planetary cores with initially 20 Earth masses implanted at different positions in the protoplanetary disc. The opacity needed for envelope contraction (Eq. (5)) directly follows the midplane opacity of the disc (Fig. 5), where the grain size distribution was derived using α = 0.001 and u_f = 1 m s⁻¹. The inclusion of carbon grains and also the water-to-silicate ratio matters for the opacities and thus envelope contraction rates.