SIMBAD references

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 P ≡ P₂/P₁ ≳ 2.3(P₁/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 three days over the number of multiple candidate systems with similar periods–perhaps a small-planet counterpart to the hot Jupiters.