Kepler Multi-Planet Systems Exhibit Unexpected Intra-system Uniformity in Mass and Radius

The widespread prevalence of close-in, nearly coplanar super-Earth- and sub-Neptune-sized planets in multiple-planet systems was one of the most surprising results from the Kepler mission. By studying a uniform sample of Kepler "multis" with mass measurements from transit timing variations (TTVs), we show that a given planetary system tends to harbor a characteristic type of planet. That is, planets in a system have both masses and radii that are far more similar than if the system were assembled randomly from planets in the population. This finding has two important ramifications. First, the large intrinsic compositional scatter in the planet mass-radius relation is dominated by system-to-system variance rather than intra-system variance. Second, if provided enough properties of the star and primordial protoplanetary disk, there may be a substantial degree of predictability in the outcome of the planet formation process. We show that stellar mass and metallicity account for of order $20\%$ of the variation in outcomes; the remainder is as-yet unknown.