Planetesimal-Driven Instabilities in Resonant Chains of Cold Neptunes and Their Dynamical Outcomes

TL;DR

This study uses N-body simulations to show that planetesimal disks can disrupt resonant chains of cold Neptunes, leading to dynamical instabilities, orbital rearrangements, and potential formation of hot Neptunes, explaining observed planetary system features.

Contribution

It demonstrates that modest planetesimal disks can induce delayed instabilities in resonant Neptune systems, providing a new pathway for hot Neptune formation and system evolution.

Findings

Planetesimal disks of 1-4% planetary mass disrupt resonant chains.

Instabilities cause orbital rearrangements and planet loss.

Many planets are scattered inward, some undergoing tidal capture or disruption.

Abstract

Cold Neptunes and sub-Neptunes are among the most common products of planet formation and likely dominate the angular-momentum budgets in most planetary systems, yet their dynamical impact on planetary architectures remains poorly understood. Using N-body simulations, we investigate the evolution of multi-Neptune systems assembled into resonant chains during the gas-disk phase and later coupled to remnant planetesimal disks. We show that planetesimal disks containing $\simeq 1$-$4\%$ of the planetary mass efficiently disrupt resonant chains and trigger global dynamical instabilities on timescales of $1~\mathrm{Myr}$-$1~\mathrm{Gyr}$, providing a pathway for delayed instability long after gas-disk dispersal, albeit with instability timescales that are highly sensitive to disk mass. The ensuing instability drives large-scale orbital rearrangement and loss of planets through collisions, tidal disruption, and ejections. Notably, in most systems at least one planet is scattered inward to $\sim 0.1~\mathrm{au}$ on $\sim 10$-$100$ Myr timescales (for $\sim 5$-$50\; M_\oplus$ planets) following instability onset, with a substantial fraction undergoing tidal capture or disruption. This tidal capture can provide a natural pathway to hot Neptune formation, while compact inner chains, if present, would be destroyed on $\sim 100~\mathrm{Myr}$ timescales by cold sub-Neptunes, naturally explaining the observed decline in the resonant fraction. We argue that the predictions of our model, which yields mass-segregated planets and corresponding relative abundances of cold, wide-orbit, and free-floating planets, can be tested by ongoing and upcoming microlensing surveys.