Long-Term Evolution of Close-in Sub-Neptunes and Outer Planetary Embryos: Atmospheric Mass Loss and Origin of Planets Inside and Outside the Radius Gap

TL;DR

This study uses N-body simulations to explore how collisions between close-in sub-Neptunes and outer planetary embryos lead to atmospheric loss, potentially explaining the observed radius gap and the rarity of high-atmosphere planets.

Contribution

It provides a detailed analysis of impact-induced atmospheric loss in multi-planet systems, highlighting the role of high-eccentricity embryos in shaping planetary radii.

Findings

Collisions cause 15-30% atmospheric loss per event.

Multiple collisions can reduce atmospheres to about one-third.

Impacts can explain the planetary radius gap and the scarcity of high-atmosphere sub-Neptunes.

Abstract

As a byproduct of sub-Neptune formation, planetary embryos with high eccentricity can remain in outer orbits, near 1 au from the star. In this work, we investigate the long-term evolution of systems consisting of close-in sub-Neptunes (SNs) and outer high-eccentricity embryos. Our analysis focuses on collisions between SNs and embryos, particularly their atmospheric mass loss. We performed N-body simulations for various initial eccentricities and numbers of embryos. We analyzed the impact-induced atmospheric loss using post-processing methods, finding that the embryos and SNs collide at high speeds on timescales of several million years, leading to the loss of the SNs' atmospheres. Depending on the embryos' eccentricity and the orbital radius of the SNs, the impact velocity can be quite high, ranging from 2 to 5 times the escape velocity. On average, about 15%-30% of the atmosphere is dissipated per collision, so after 3-6 collisions, the atmospheric mass of an SN is reduced to about 1/3 of its initial value. Collisions between SNs and embryos can thus explain the presence of planets within the radius gap. Depending upon the initial eccentricity and the number of remaining embryos, additional collisions can occur, potentially accounting for the formation of the radius gap. This study also indicates that collisions between remaining embryos and SNs may help to explain the observed rarity of SNs with atmospheric mass fractions greater than 10%, commonly termed the "radius cliff."