Architectures of Exoplanetary Systems. IV. A Multiplanet Model for Reproducing the Radius Valley and Intrasystem Size Similarity of Planets around Kepler’s FGK Dwarfs

The Kepler-observed distribution of planet sizes has revealed two distinct patterns: (1) a radius valley separating super-Earths and sub-Neptunes and (2) a preference for intrasystem size similarity. We present new models for the exoplanet population observed by Kepler, which are hybrids of a clustered multiplanet model in which the orbital architectures are set by the angular momentum deficit stability and a joint mass–radius–period model involving envelope mass loss driven by photoevaporation. We find that the models that produce the deepest radius valleys have a primordial population of planets with initial radii peaking at ∼2.1 R_⊕, which is subsequently sculpted by photoevaporation into a bimodal distribution of final planet radii. The hybrid models require strongly clustered initial planet masses in order to match the distributions of the size similarity metrics. Thus, the preference for intrasystem radius similarity is well explained by a clustering in the primordial mass distribution. The hybrid models also naturally reproduce the observed radius cliff (steep drop-off beyond ∼2.5 R_⊕). Our hybrid models are the latest installment of the SysSim forward models, and are the first multiplanet models capable of simultaneously reproducing the observed radius valley and the intrasystem size similarity patterns. We compute occurrence rates and fractions of stars with planets for a variety of planet types, and find that the occurrence of Venus- and Earth-like planets drops by a factor of ∼2–4 for the hybrid models compared to previous clustered models in which there is no envelope mass loss.