Presentation #501.03 in the session Architectures 2.
Although the architectures of compact multiple-planet systems are well-studied, little has been examined about their “outer edges”, or the locations of the outermost planets in these systems. 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 periods and radii dictated by the expected “peas-in-a-pod” patterns. 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 transit detections. We find that at least ~40% of Kepler compact multis would be expected to show an additional transiting and detected planet orbiting beyond the known planets, constituting a ~7-sigma discrepancy with the lack of such detections. These results can be explained by a decrease in planet occurrence in the outer regions of compact multis (a.k.a. an “edge-of-the-multis”). However, our results can also be reconciled by a significant breakdown of the “peas-in-a-pod” patterns at longer periods, in the form of systematically smaller planet radii or larger period ratios. We outline future observations that can help discriminate these possibilities, and we discuss the implications of our results for planet formation theory.