Dust rings trap protoplanets on eccentric orbits and get consumed by them

TL;DR

This study uses 3D simulations to show dust rings can trap protoplanets on eccentric orbits, enabling them to efficiently accrete mass and potentially become gas giants, with stable trapping mechanisms that are robust against uncertainties.

Contribution

It reveals two stable eccentric trapping orbits for protoplanets near dust rings, demonstrating a new mechanism for planet growth and migration inhibition.

Findings

Protoplanets can be trapped on stable eccentric orbits near dust rings.

Outer orbit trapping enables rapid mass growth to runaway gas accretion.

Inner orbit trapping results in slower growth, possibly remaining as super-Earths.

Abstract

We study the orbital evolution and mass growth of protoplanets with masses $M \in [0.1-8]$~M$_\oplus$ in the vicinity of a dusty ring, using three-dimensional numerical simulations with a two-fluid model and nested-meshes. We find two stable, eccentric orbits that lock the planet in the ring vicinity, thereby inhibiting its migration and allowing it to accrete dust from the ring. One of these orbits has an eccentricity comparable to the aspect ratio of the gaseous disc and has its periastron within the ring, enabling intermittent accretion during each pass. The other orbit has a smaller eccentricity and an apoastron slightly inside the ring. A planet locked at the outer orbit efficiently accretes from the ring and can reach the critical mass for runaway gas accretion on timescales $\gtrsim 10^5$ yr (for a 10~M$_\oplus$ dust ring at 10~au) while a planet locked at the inner orbit has a slower growth and might not supersede the super-Earth stage over the disc lifetime. While in our runs a low-mass embryo forming within the ring eventually joins the outer orbit, it is likely that the path taken depends on the specific details of the ring. The trapping on the outer orbit arises from an intermittent, strong thermal force at each passage through the ring, where the accretion rate spikes. It is insensitive to uncertainties that plague models considering planets trapped on circular orbits in rings. It is highly robust and could allow a growing planet to follow an expanding ring over large distances.