Peas-in-a-pod across the Radius Valley: Rocky Systems Are Less Uniform in Mass but More Uniform in Size and Spacing

Abstract

The ubiquity of “peas-in-a-pod” architectural patterns and the existence of the radius valley each presents a striking population-level trend for planets with R p ≤ 4 R ⊕ that serves to place powerful constraints on the formation and evolution of these subgiant worlds. As it has yet to be determined whether the strength of this peas-in-a-pod uniformity differs on either side of the radius valley, we separately assess the architectures of systems containing only small ( R p ≤ 1.6 R ⊕ ), rocky planets from those harboring only intermediate-sized (1.6 R ⊕ < R p ≤ 4 R ⊕ ), volatile-rich worlds to perform a novel statistical comparison of intra-system planetary uniformity across compositionally distinct regimes. We find that, compared to their volatile-rich counterparts, rocky systems are less uniform in mass (2.6 σ ) but more uniform in size (4.0 σ ) and spacing (3.0 σ ). We provide further statistical validation for these results, demonstrating that they are not substantially influenced by the presence of mean-motion resonances, low-mass host stars, alternative bulk compositional assumptions, sample size effects, or detection biases. We also obtain tentative evidence (>2 σ significance) that the enhanced size uniformity of rocky systems is dominated by the presence of super-Earths (1 R ⊕ ≤ R p ≤ 1.6 R ⊕ ), while their enhanced mass diversity is driven by the presence of sub-Earth ( R p < 1 R ⊕ ) worlds.