A lack of short-period multiplanet systems with close-proximity pairs and the curious case of Kepler 42

TL;DR

This paper investigates the scarcity of short-period multiplanet systems with close pairs, highlighting Kepler-42 as a rare example, and analyzes the statistical distribution of such systems.

Contribution

It provides the first statistical analysis predicting the existence of ~17 such systems and explores the reasons behind their scarcity, including a potential small-planet counterpart to hot Jupiters.

Findings

Kepler-42 is a unique example of a short-period multiplanet system.

Predicted ~17 such systems should exist based on statistical analysis.

An excess of short-period single small planets suggests a possible small-planet analog to hot Jupiters.

Abstract

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_2/P_1 >~ 2.3 (P_1/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.