Schematic representation of the proposed evolutionary model for the HR 8799 system. The pair of curves around planets represents the planet under the influence of the primordial gas disk, which is represented as the gray region. By contrast, the gas-free region is colored in white. Finally, the black dots represent the planetesimal bodies. In Stage 0, the protoplanets form and migrate inward until they are stopped at the inner edge and trapped in an MMR. In Stage 1, the three inner planets migrate outward, which compels the outermost planet to also migrate outward due to their MMR chain, even though it would otherwise move inward due to gas drag. This outward movement displaces the inner edge of the planetesimal disk. In Stage 2, the three inner planets cease their outward migration, allowing the outermost planet to resume migrating inward, although this movement is marginal. At this point, the system achieves its current architecture.