Enhanced Size Uniformity for Near-resonant Planets

Abstract

Super-Earths within the same close-in, compact planetary system tend to exhibit a striking degree of uniformity in their radius, mass, and orbital spacing, and this “peas-in-a-pod” phenomenon itself serves to provide one of the strongest constrains on planet formation at large. While it has been recently demonstrated from independent samples that such planetary uniformity occurs for both configurations near and distant from mean motion resonance, the question thus remains if the strength of this uniformity itself differs between near-resonant and nonresonant configurations such that the two modes may be astrophysically distinct in their evolution. We thus provide in this work a novel comparative size uniformity analysis for 48 near-resonant and 251 nonresonant multiplanet systems from the California Kepler Survey catalog, evaluating uniformity both across systems and between planetary pairs within the same system. We find that while multiplanet configurations exhibit strong peas-in-a-pod size uniformity regardless of their proximity to resonance, near-resonant configurations display enhanced intra-system size uniformity as compared to their analogous nonresonant counterparts at the level of both entire systems and subsystem planetary pairs and chains. These results are broadly consistent with a variety of formation paradigms for multiple-planet systems, such as convergent migration within a turbulent protoplanetary disk or planet–planet interactions incited by postnebular dynamical instabilities. Nevertheless, further investigation is necessary to ascertain whether the nonresonant and near-resonant planetary configurations respectively evolve via a singular process or mechanisms that are dynamically distinct.

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

“…Given that all Kepler planetary orbital periods are precisely determined from transit photometry such that associated fractional uncertainties are themselves generally of order δP/P 10 −5 (Lissauer et al 2023), we do not adopt for our spacing uniformity analysis the uncertainty propagation procedure utilized in Sections 4 and 5. Instead, we shall consider instead the following null hypothesis testing framework put forth by Goyal et al (2023): we adopt the null hypothesis that the high-N CKS rocky and volatile-rich samples exhibit no difference in the degree of their intra-system spacing uniformity and thus compare the mean-spacing Gini   of the 13 high-N CKS rocky systems to a control distribution of d      calculated for 10 5 subsamples of 13 systems drawn randomly (without replacement) from the 30 high-N CKS volatile-rich systems. From this procedure, we obtain for our 13 high-N CKS rocky systems and our 30 high- , indicating a 3.01σ discrepancy corresponding to enhanced spacing uniformity for rocky systems.…”

“…In accord with the analyses of Goyal & Wang (2022) and Goyal et al (2023), we adopt the adjusted Gini index (Deltas 2003), a common economic measure of wealth or income inequality in a given population, as our primary metric for the assessment of intra-system mass and spacing uniformity. The adjusted Gini index can be expressed mathematically as…”

“…Given that planetary systems containing at least one pair close to first-order MMRs exhibit enhanced size uniformity compared to systems that are are entirely nonresonant (Goyal et al 2023), it remains to be seen if such effects may confound any emergent discrepancies in planetary size or mass uniformity between different compositional regimes. Following the methodology of Goyal et al (2023) we search for planetary pairs in close proximity to first-order MMRs using the ζ 2,1 parameter from Fabrycky et al (2014):…”

Evidence for Low-level Dynamical Excitation in Near-resonant Exoplanet Systems*

“…The dynamical histories of planetary systems can, to some extent, be reconstructed through their current orbital demographics. Near-resonant systems, in which two or more planets exhibit near-exact integer ratio commensurabilities of their orbital periods, offer an especially well-constrained lens into the evolution of planetary systems (e.g., Goldreich & Sciama 1965;Lee & Peale 2002;Millholland et al 2018;Goyal et al 2023). In these cases, formation models must jointly account for both the systems' near-resonant configurations and their currently observed orbital geometries.…”