Histogram of the dust surface density, normalized to that at t = 0, around a pair of planets trapped in 2:1 resonance The plot refers to the same case as in Fig. 10, at time t = 70 Kyr (see bottom panel of that figure). The green filled circles give the positions of the planets. The different line colors correspond to different grain sizes, from 10 μm to 1 cm. Only minor differences are observed among the various sizes because all grains are well coupled to the gas.