Size Evolution and Orbital Architecture of KEPLER Small Planets through Giant Impacts and Photoevaporation

Abstract The KEPLER transit survey with follow-up spectroscopic observations has discovered numerous small planets (super-Earths/sub-Neptunes) and revealed intriguing features of their sizes, orbital periods, and their relations between adjacent planets. The planet size distribution exhibits a bimodal distribution separated by a radius gap at around 1.8 Earth radii. Besides, these small planets within multiple planetary systems show that adjacent planets are similar in size and their period ratios of adjacent planet pairs are similar as well, a phenomenon often dubbed as peas-in-a-pod in the exoplanet community. While the radius gap has been predicted and theorized for years, whether it can be relevant to the orbital architecture peas-in-a-pod is physically unknown. For the first time, we attempted to model both features together through planet formation and evolution processes involving giant impacts and photoevaporation. We showed that our model is generally consistent with the KEPLER results but with a smaller radius gap. The impact of Kubyshikina’s model for photoevaporation on our model is discussed.