# Multiplanet systems in inviscid discs can avoid forming resonant chains

## TL;DR

This paper shows that in inviscid protoplanetary discs, planet migration often results in unstable, non-resonant systems, contrasting with viscous discs which tend to form resonant chains, highlighting the importance of disc viscosity in planetary system architecture.

## Contribution

It demonstrates that inviscid discs can produce stable, non-resonant planetary systems, challenging the expectation of resonant chains from convergent migration.

## Key findings

- Inviscid discs often lead to dynamically unstable, non-resonant systems.
- Massive planets can disrupt resonance formation through vortex-driven perturbations.
- Stable, closely packed, non-resonant systems can form in inviscid discs.

## Abstract

Convergent migration involving multiple planets embedded in a viscous protoplanetary disc is expected to produce a chain of planets in mean motion resonances, but the multiplanet systems observed by the Kepler spacecraft are generally not in resonance. We demonstrate that under equivalent conditions, where in a viscous disc convergent migration will form a long-term stable system of planets in a chain of mean motion resonances, migration in an inviscid disc often produces a system which is highly dynamically unstable. In particular, if planets are massive enough to significantly perturb the disc surface density and drive vortex formation, the smooth capture of planets into mean motion resonances is disrupted. As planets pile up in close orbits, not protected by resonances, close encounters increase the probability of planet-planet collisions, even while the gas disc is still present. While inviscid discs often produce unstable non-resonant systems, stable, closely packed, non-resonant systems can also be formed. Thus, when examining the expectation for planet migration to produce planetary systems in mean motion resonances, the effective turbulent viscosity of the protoplanetary disc is a key parameter.