Planets in the Radius Gap Lack the Peas-in-a-pod Size Correlation

Abstract Planets in compact multitransiting systems tend to exhibit self-similarity with their neighbors, a phenomenon commonly called “peas-in-a-pod.” Previous studies have identified that this self-similarity appears independently among super-Earths and sub-Neptunes orbiting the same star. In this study, we investigate whether the peas-in-a-pod phenomenon holds for planets in the radius valley between these two categories (located at ∼1.8 R ⊕ ). Employing the Kepler sample of planets in multitransiting systems, we construct a difference-in-differences test that compares the observed fraction of size-similar adjacent pairs to the fraction expected from the underlying radius distribution alone, computed independently for valley-inclusive and valley-exclusive pairs. We find that nonvalley pairs exhibit a 1.87× enhancement of size-similar pairs above the baseline, consistent with the well-established peas-in-a-pod phenomenon. Pairs involving a radius-valley planet show no such enhancement, and we exclude at p = 0.001 the hypothesis that the same size-similarity mechanism operates at the same strength for valley-inclusive pairs. The observed fraction of size-similar valley-inclusive pairs is consistent with independent draws from the radius distribution, with no additional intrasystem correlation. We further compare the period-ratio distributions for the two pair classes. While globally indistinguishable (KS p = 0.848), valley-inclusive pairs cluster near the 3:2 mean-motion resonance at more than twice the rate of the parent population, while avoiding the tightest orbital spacings entirely. The convergence of disrupted size-similarity and anomalous resonance architecture, together with independently measured elevated eccentricities among valley planets, is consistent with a stochastic process such as late-stage giant impacts contributing to the population of planets in the radius valley.