Peas-in-a-Pod: A Review on the Observed Patterns of Intra-System Uniformity

One of the most important developments in exoplanet science in the past decade is the discovery of multi-planet systems with sub-Neptune-sized planets inside of ~1 AU. The architectures of these so-called “compact multis” often display a remarkable degree of uniformity: the planets have nearly equal sizes and masses, regular orbital spacing, low eccentricities, and small mutual inclinations. In particular, the planet radii, masses, and period ratios within a given system are significantly more uniform than expected if systems were assembled from planets drawn randomly from the population and subjected to the appropriate detection biases. This uniformity stands in sharp contrast to the diverse nature of the exoplanet sample considered as a whole, as well as our inner solar system. In this talk, we will review the observational evidence for intra-system uniformity and explore the global system properties where uniformity is prevalent.