Breakdown of planetary systems in embedded clusters

Abstract

ABSTRACT We report the first simulations of planetary system dynamics as affected by an embedded cluster environment. Such environments are generally believed to be relevant for the large majority of newborn stars of solar type. Moreover, our cluster model is more realistic than in previous work. We focus on a giant planet system with five members, which represents a likely precursor of our solar system. Our main result is that the perturbing effects of close encounters with cluster stars trigger dynamical chaos leading to breakdown of the system with a significant probability, especially if the natal gas discs are short-lived and the clusters are highly concentrated. When breakdown occurs, all planets except Jupiter suffer a large risk of being ejected from the system or extracted into distant orbits with semimajor axes of hundreds or thousands of astronomical units. This is consistent with recent estimates of a large abundance of low-mass, free-floating planets. We demonstrate a possibility for Jupiter and Saturn to evolve into hot Jupiter orbits by tidal circularization during the chaotic evolution. Even so, the low occurrence rate of this outcome indicates that the real hot Jupiters in general have an origin unrelated to dynamical evolution in birth clusters.