Left: particles’ radial velocities as a function of their Stokes numbers (black dots) at 15 R_Jup from a Jupiter mass planet in the midplane of a CPD with ﹩{\dot{M}}_{p}={10}^{-7}\,{M}_{\mathrm{Jup}}\,{\mathrm{yr}}^{-1}﹩ and α = 10⁻³. The solid line shows the solution of the analytical formula given by Equation (17), which fits well the results of our integration. Small dust grains with sizes smaller than ∼10⁻³ m have a slightly positive velocity, which is that of the gas at the midplane (v_r,g ≃ 0.15 m s⁻¹). Overall, there is more than one order of magnitude difference between the velocity of pebbles (solids with St ∼ 1) and those of the larger (St ≫ 1) and smaller (St ≪ 1) particles. Right: correspondance between the Stokes number and the size of the particles.