Distribution of the period ratio between pairs of planets close to the 2:1 and 3:2 mean-motion resonances. The distributions around both resonances are accumulated to obtain a more meaningful set of systems. These statistics are obtained from the Q1-Q16 KOI catalog (Batalha et al. 2013). The origin of the x-axis is the exact commensurability of the periods (resonant systems) and is highlighted with a red strip. Negative values correspond to internal circulation (P₂/P₁< (p + 1) /p), positive values to external circulation (P₂/P₁> (p + 1) /p). We observe a strong excess of systems in external circulation, with P₂/P₁ − (p + 1) /p ≈ 2 × 10^-2.