Phase portraits similar to those of Fig. 7 but to which we have added a second perturbing planet (representing the disk) apsidally aligned with the first one (Fom b). The second planet is assumed to orbit at the same semi-major axis as the disk particle. The important parameter is the mass ratio ρ between the two planets. Left plot: ρ = 0.1, i.e., a disk 10 times less massive than Fom b; Middle plot: ρ = 1, disk and Fom b have equal masses; Right plot: ρ = 10, i.e., a disk 10 times more massive than Fom b.