The Architecture of Multiplanet Systems as a Tracer of Their Formation Mechanisms

Exoplanets observed by the Kepler telescope exhibit a bimodal, radius distribution, which is known as the radius gap. We explore an origin of the radius gap, focusing on multiplanet systems. Our simple theoretical argument predicts that type I planetary migration produces different configurations of protoplanets with different masses and such different configurations can result in two distinguishable populations of small-sized multiplanet systems. We then perform an observational analysis to verify this prediction. In the analysis, multiple Kolmogorov–Smirnov tests are applied to the observed systems, using the statistical measures that are devised to systematically characterize the properties of multiplanet systems. We find with 99.5% confidence that the observed, small-sized multiplanet systems are divided into two distinct populations. The distinction likely originates from different spatial distributions of protoplanets, which are determined by type I migration and subsequently trigger giant impact. We also show that these distinct populations are separated around the radius gap when the gas surface density of protoplanetary disks is ∼102 g cm−2 in the vicinity of the host stars. This work therefore emphasizes the importance of planetary migration and the inner disk properties.