The architecture of multi-planet systems as a tracer of their formation mechanisms

Exoplanets observed by the Kepler telescope exhibit a bi-modal, radius distribution, which is knownas the radius gap. We explore an origin of the radius gap, focusing on multi-planet systems. Oursimple theoretical argument predicts that type I planetary migration produces different configurationsof protoplanets with different masses and such different configurations can result in two distinguishable populations of small-sized multi-planet systems. We then perform an observational analysis toverify this prediction. In the analysis, multiple Kolmogorov–Smirnov tests are applied to the observedsystems, using the statistical measures that are devised to systematically characterize the propertiesof multi-planet systems. We find with 99.5% confidence that the observed, small-sized multi-planetsystems 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 triggergiant impact. We also show that these distinct populations are separated around the radius gap whenthe gas surface density of protoplanetary disks is ∼ 102 g cm−2in the vicinity of the host stars. Thiswork therefore emphasizes the importance of planetary migration and the inner disk properties.