SIMBAD references

We investigate the distributions of the orbital period ratios of adjacent planets in high-multiplicity Kepler systems (four or more planets) and low-multiplicity systems (two planets). Modelling the low-multiplicity sample as essentially equivalent to the high-multiplicity sample, but with unobserved intermediate planets, we find some evidence for an excess of planet pairs between the 2:1 and 3:1 mean-motion resonances in the low-multiplicity sample. This possible excess may be the result of strong dynamical interactions near these or other resonances or it may be a byproduct of other evolutionary events or processes such as planetary collisions. Three-planet systems show a significant excess of planets near the 2:1 mean-motion resonance that is not as prominent in either of the other samples. This observation may imply a correlation between strong dynamical interactions and observed planet number - perhaps a relationship between resonance pairs and the inclinations or orbital periods of additional planets. The period ratio distributions can also be used to identify targets to search for missing planets in the each of the samples, the presence or absence of which would have strong implications for planet formation and dynamical evolution models.