  A transiting planet exhibits sinusoidal transit-time-variations (TTVs) if
perturbed by a companion near a mean-motion-resonance (MMR). We search for
sinusoidal TTVs in more than 2600 Kepler candidates, using the publicly
available Kepler light-curves (Q0-Q12). We find that the TTV fractions rise
strikingly with the transit multiplicity. Systems where four or more planets
transit enjoy roughly five times higher TTV fraction than those where a single
planet transits, and about twice higher than those for doubles and triples. In
contrast, models in which all transiting planets arise from similar dynamical
configurations predict comparable TTV fractions among these different systems.
One simple explanation for our results is that there are at least two different
classes of Kepler systems, one closely packed and one more sparsely populated.
