Peas-in-a-Pod Across the Radius Valley: Rocky Systems are Less Uniform in Mass but More Uniform in Size and Spacing

TL;DR

This study compares the architectures of rocky and volatile-rich planetary systems, revealing rocky systems are less uniform in mass but more uniform in size and spacing, providing insights into planet formation.

Contribution

It presents a novel statistical analysis of intra-system planetary uniformity across different planetary compositions, highlighting differences on either side of the radius valley.

Findings

Rocky systems are less uniform in mass (2.6σ)

Rocky systems are more uniform in size (4.0σ)

Rocky systems are more uniform in spacing (3.0σ)

Abstract

The ubiquity of "peas-in-a-pod" architectural patterns and the existence of the radius valley each present a striking population-level trend for planets with $R_{p} \leq 4 R_{\oplus}$ 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 ($R_{p} \leq 1.6 R_{\oplus}$), rocky planets from those harboring only intermediate-size ($1.6 R_{\oplus} < R_{p} \leq 4 R_{\oplus}$), 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 ($2.6\sigma$), but more uniform in size ($4.0\sigma$) and spacing ($3.0\sigma$). 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 ($>2 \sigma$ significance) that the enhanced size uniformity of rocky systems is dominated by the presence of super-Earths ($1 R_{\oplus} \leq R_{p} \leq 1.6 R_{\oplus}$), while their enhanced mass diversity is driven by the presence of sub-Earth ($R_{p} < 1 R_{\oplus}$) worlds.