Singles in Disguise: Kepler's Low-SNR Single-Transiting Systems as Incompletely Detected Multis

Abstract

NASA's Kepler mission found more systems with one transiting planet (``singles'') than expected from those with several (``multis''). This excess was first taken as evidence for a Kepler dichotomy -- a second population of intrinsically single or highly inclined systems -- but later work argued that selection effects acting on a single population can reproduce it. Singles also show a puzzling excess of low signal-to-noise ratio (SNR) detections relative to multis. The question arises whether this excess reflects false positives, a selection effect, or an inherent population difference. Here we show, using the ExoMOD population synthesis model and the Kepler DR25 catalog, that multis drawn from the same parent population as singles have shorter mean orbital periods, and thus higher-SNR detections on average. Detecting several planets in one system favors compact architectures, so long-period, low-SNR detections are preferentially cataloged as singles, most of them intrinsically multi-planet systems whose other planets went undetected. The low-SNR excess of singles therefore requires neither false positives nor a Kepler dichotomy but rather only the geometry and period sensitivity of transit detection.