Evolution in the period ratios plane. The points are colour-coded according to their time before the close encounter. We note that the system spends almost all of its time very close to its starting location as shown in Figs. 1 and 2. Green oblique lines correspond the loci of the zeroth-order three-planet MMRs (see Sect. 4.1). Chirikov’s diffusion is expected to occur perpendicularly to the network direction, along the dashed orange line. The extent of the adjacent first-order two-planet MMRs, 7:6 and 6:5, is plotted in grey. The width is computed for circular orbits. The second-order resonance 13:11 is plotted in red. Once the system enters the two-planet resonance network, the diffusion is much more rapid.