Edge-of-the-Multis: Evidence for a Transition in the Outer Architectures of Compact Multi-Planet Systems

TL;DR

This study investigates the outer boundaries of compact multi-planet systems, providing evidence that these systems truncate or change in structure beyond certain orbital distances, challenging existing formation models.

Contribution

It introduces a novel method to estimate mutual inclination dispersion and assesses the detectability of hypothetical outer planets, revealing a discrepancy with observations that suggests a truncation or pattern breakdown.

Findings

Over 35% of Kepler systems should have additional outer planets if patterns continue.

Observed lack of outer planets indicates a truncation or change in system architecture.

Results challenge the assumption that 'peas-in-a-pod' patterns extend to larger orbital separations.

Abstract

Although the architectures of compact multiple-planet systems are well-characterized, there has been little examination of their "outer edges", or the locations of their outermost planets. Here we present evidence that the observed high-multiplicity Kepler systems truncate at smaller orbital periods than can be explained by geometric and detection biases alone. To show this, we considered the existence of hypothetical planets orbiting beyond the observed transiting planets with properties dictated by the "peas-in-a-pod" patterns of intra-system radius and period ratio uniformity. We evaluated the detectability of these hypothetical planets using (1) a novel approach for estimating the mutual inclination dispersion of multi-transiting systems based on transit chord length ratios and (2) a model of transit probability and detection efficiency that accounts for the impacts of planet multiplicity on completeness. Under the assumption that the "peas-in-a-pod" patterns continue to larger orbital separations than observed, we find that $\gtrsim35\%$ of Kepler compact multis should possess additional detected planets beyond the known planets, constituting a $\sim7\sigma$ discrepancy with the lack of such detections. These results indicate that the outer ($\sim100-300$ days) regions of compact multis experience a truncation (i.e. an "edge-of-the-multis") or a significant breakdown of the "peas-in-a-pod" patterns, in the form of systematically smaller radii or larger period ratios. We outline future observations that can distinguish these possibilities, and we discuss implications for planet formation theories.