Diversities and similarities exhibited by multi-planetary systems and their architectures

I. Orbital spacings

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Chalmersplatsen 4, 412 58 Gothenburg, Sweden
² Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands

^★ Corresponding author; muresan@chalmers.se

Received: 2 July 2024
Accepted: 27 October 2024

Abstract

The rich diversity of multi-planetary systems and their architectures is greatly contrasted by the uniformity exhibited within many of these systems. Previous studies have shown that compact Kepler systems tend to exhibit a peas-in-a-pod architecture: Planets in the same system tend to have similar sizes and masses and be regularly spaced in orbits with low eccentricities and small mutual inclinations. This work extends on previous research and examines a larger and more diverse sample comprising all the systems with a minimum of three confirmed planets, resulting in 282 systems and a total of 991 planets. We investigated the system architectures, focusing on the orbital spacings between adjacent planets as well as their relationships with the planets’ sizes and masses. We also quantified the similarities of the sizes, masses, and spacings of planets within each system, conducting both intra- and inter-system analyses. Our results corroborate previous research showing that planets orbiting the same star tend to be regularly spaced and that pairs of adjacent planets with radii < 1 R_⊕ predominantly have orbital period ratios (PRs) smaller than two. In contrast to other studies, we identified a significant similarity of adjacent orbital spacings not only at PRs < 4 but also at 1.17 < PRs < 2662. For the systems with transiting planets, we additionally found that the reported correlation between the orbital PRs and the average sizes of adjacent planets disappears when planet pairs with R < 1 R_⊕ are excluded. Furthermore, we examined the data for possible correlations between the intra-system dispersions of the orbital spacings and those of the planetary radii and masses. Our findings indicate that these dispersions are uncorrelated for the systems in which all the pairs of adjacent planets have PRs < 6, and even for the compact systems where all PRs < 2. Notably, planets in the same system can be similarly spaced even if they do not have similar masses or sizes.