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 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 . 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 three days over the number of multiple candidate systems with similar periods - perhaps a small-planet counterpart to the hot Jupiters. © 2013. The American Astronomical Society. All rights reserved.
Steffen, J. H., & Farr, W. M. (2013). A lack of short-period multiplanet systems with close-proximity pairs and the curious case of kepler-42. Astrophysical Journal Letters, 774(1). https://doi.org/10.1088/2041-8205/774/1/L12