Architectures of Exoplanetary Systems: A Multi-planet Model for Reproducing the Radius Valley and Intra-system Size Similarity of the Kepler Planets

The single and multi-planet systems discovered by NASA's Kepler mission provide crucial insights into the architectures and correlations within planetary systems, which in turn offer clues into their formation and evolution histories. The observed distribution of planet sizes from the Kepler planet catalog have revealed two distinct patterns: (1) a radius valley separating super-Earths and sub-Neptunes and (2) a preference for intra-system size similarity. I will present a new model for the exoplanet population observed by Kepler, which combines a multi-planet model in which the orbital architectures are set by the angular momentum deficit (AMD) stability with a joint mass-radius-period model involving envelope mass-loss driven by photo-evaporation. This "hybrid" model is capable of reproducing the observed radius valley given appropriate choices of the model parameters. The models that produce the deepest radius valleys have a primordial population of planets with initial radii peaking at ~2.1 Earth radii, which is subsequently sculpted by photo-evaporation into a bimodal distribution of final planet radii. I will show that the hybrid model requires strongly clustered initial planet masses in order to match the observed distributions of the size similarity metrics from Kepler's multi-planet systems. Thus, the preference for correlated planet radii within the same system is well explained by a clustering in the primordial mass distribution. I will also show that the hybrid model naturally reproduces the "radius cliff" (the steep drop-off beyond ~2.5 Earth radii). This hybrid model is the first multi-planet model capable of simultaneously reproducing the observed radius valley and the intra-system size similarity patterns. Finally, I will discuss the model predictions for the occurrence rates of various types of planets, including Venus and Earth-sized analogs.