We analyse the distribution of known multi-planet systems (N ≥ 3) in the plane of mean-motion ratios, and compare it with the resonance web generated by two-planet mean-motion resonances (2P-MMR) and pure 3-planet commensurabilities (pure 3P-MMR). We find intriguing evidence of a statistically significant correlation between the observed distribution of compact low-mass systems and the resonance structure, indicating a possible causal relation. While resonance chains such as Kepler-60, Kepler-80 and TRAPPIST-1 are strong contributors, most of the correlation appears to be caused by systems not identified as resonance chains. Finally, we discuss their possible origin through planetary migration during the last stages of the primordial disc and/or an eccentricity damping process.

No abstract