Many Kepler multiplanet systems have planet pairs near low-order , mean-motion resonances .
In addition , many Kepler multiplanet systems have planets with orbital periods less than a few days .
With the exception of Kepler-42 , however , there are no examples of systems with both short orbital periods and nearby companion planets while our statistical analysis predicts \sim 17 such pairs .
For orbital periods of the inner planet that are less than three days , the minimum period ratio of adjacent planet pairs follows the rough constraint \mathcal { P } \equiv P _ { 2 } / P _ { 1 } \gtrsim 2.3 ( P _ { 1 } / \textrm { day } ) ^ { -2 / 3 } .
This absence is not due to a lack of planets with short orbital periods .
We also show a statistically significant excess of small , single candidate systems with orbital periods below 3 days over the number of multiple candidate systems with similar periods—perhaps a small-planet counterpart to the hot Jupiters .