Planetesimals to Protoplanets II: Effect of Debris on Terrestrial Planet Formation

TL;DR

This study enhances numerical simulations of terrestrial planet formation by including dynamical friction from unresolved debris, revealing that initial debris mass influences growth modes and migration patterns of protoplanets.

Contribution

The paper introduces a new numerical model that incorporates dynamical friction from unresolved debris into planet formation simulations, enabling more accurate analysis of debris effects.

Findings

High initial debris mass (>10%) suppresses runaway growth.

Dynamical friction reduces collisional mixing and scattering.

Significant inward migration of protoplanets occurs under extreme conditions.

Abstract

In this paper we extend our numerical method for simulating terrestrial planet formation from Leinhardt and Richardson (2005) to include dynamical friction from the unresolved debris component. In the previous work we implemented a rubble pile planetesimal collision model into direct N-body simulations of terrestrial planet formation. The new collision model treated both accretion and erosion of planetesimals but did not include dynamical friction from debris particles smaller than the resolution limit for the simulation. By extending our numerical model to include dynamical friction from the unresolved debris, we can simulate the dynamical effect of debris produced during collisions and can also investigate the effect of initial debris mass on terrestrial planet formation. We find that significant initial debris mass, 10% or more of the total disk mass, changes the mode of planetesimal growth. Specifically, planetesimals in this situation do not go through a runaway growth phase. Instead they grow concurrently, similar to oligarchic growth. In addition to including the dynamical friction from the unresolved debris, we have implemented particle tracking as a proxy for monitoring compositional mixing. Although there is much less mixing due to collisions and gravitational scattering when dynamical friction of the background debris is included, there is significant inward migration of the largest protoplanets in the most extreme initial conditions.