The dynamical evolution of multi-planet systems in open clusters

TL;DR

This study investigates how frequent stellar encounters in open clusters destabilize multi-planet systems, leading to planet ejections or collisions, which may explain the observed scarcity of exoplanets in such environments.

Contribution

The paper demonstrates through simulations that stellar encounters significantly impact multi-planet systems in star clusters, revealing mechanisms for planet loss and system evolution.

Findings

Stellar encounters can cause ejections and collisions in multi-planet systems.

Outer planets are more directly affected by stellar encounters.

Inner planets can be destabilized long after encounters, leading to planet loss.

Abstract

The majority of stars form in star clusters and many are thought to have planetary companions. We demonstrate that multi-planet systems are prone to instabilities as a result of frequent stellar encounters in these star clusters much more than single-planet systems. The cumulative effect of close and distant encounters on these planetary systems are investigated using Monte Carlo scattering experiments. We consider two types of planetary configurations orbiting Sun-like stars: (i) five Jupiter-mass planets in the semi-major axis range 1-42 AU orbiting a Solar mass star, with orbits that are initially co-planar, circular, and separated by 10 mutual Hill radii, and (ii) the four gas giants of our Solar system. Planets with short orbital periods are not directly affected by encountering stars. However, secular evolution of perturbed systems may result in the ejection of the innermost planets or in physical collisions of the innermost planets with the host star, up to many thousands of years after a stellar encounter. The simulations of the Solar system-like systems indicate that Saturn, Uranus and Neptune are affected by both direct interactions with encountering stars, as well as planet-planet scattering. Jupiter, on the other hand, is almost only affected by direct encounters with neighbouring stars, as its mass is too large to be substantially perturbed by the other three planets. Our results indicate that stellar encounters can account for the apparent scarcity of exoplanets in star clusters, not only for those on wide-orbit that are directly affected by stellar encounters, but also planets close to the star which can disappear long after a stellar encounter has perturbed the planetary system.