Size evolution of close-in super-Earths through giant impacts and photoevaporation

TL;DR

This study models the size evolution of close-in super-Earths through giant impacts and photoevaporation, explaining observed size distributions and orbital configurations.

Contribution

It introduces a combined analytical and simulation approach to understand how giant impacts and photoevaporation shape super-Earth sizes and spacings.

Findings

Inner planets can lose their entire envelopes via giant impacts.

Outer planets retain their initial envelopes due to fewer impacts.

Simulated size and spacing distributions match observed planetary systems.

Abstract

The Kepler transit survey with follow-up spectroscopic observations has discovered numerous super-Earth sized planets and revealed intriguing features of their sizes, orbital periods, and their relations between adjacent planets. For the first time, we investigate the size evolution of planets via both giant impacts and photoevaporation to compare with these observed features. We calculate the size of a protoplanet, which is the sum of its core and envelope sizes, by analytical models. $N$-body simulations are performed to evolve planet sizes during the giant impact phase with envelope stripping via impact shocks. We consider the initial radial profile of the core mass and the initial envelope mass fractions as parameters. Inner planets can lose their whole envelopes via giant impacts, while outer planets can keep their initial envelopes since they do not experience giant impacts. Photoevaporation is simulated to evolve planet sizes afterward. Our results suggest that the period-radius distribution of the observed planets would be reproduced if we perform simulations in which the initial radial profile of the core mass follows a wide range of power-law distributions and the initial envelope mass fractions are $\sim0.1$. Moreover, our model shows that the adjacent planetary pairs have similar sizes and regular spacings, with slight differences from detailed observational results such as the radius gap.