Survivability of planetary systems in young and dense star clusters

Abstract: We perform a simulation using the Astrophysical Multipurpose Software Environment of the Orion Trapezium star cluster in which the evolution of the stars and the dynamics of planetary systems are taken into account. The initial conditions from earlier simulations were selected in which the size and mass distributions of the observed circumstellar disks in this cluster are satisfactorily reproduced. Four, five, or size planets per star were introduced in orbit around the 500 solar-like stars with a maximum orbital separation of 400au. Our study focuses on the production of free-floating planets. A total of 357 become unbound from a total of 2522 planets in the initial conditions of the simulation. Of these, 281 leave the cluster within the crossing timescale of the star cluster; the others remain bound to the cluster as free-floating intra-cluster planets. Five of these free-floating intra-cluster planets are captured at a later time by another star. The two main mechanisms by which planets are lost from their host star, ejection upon a strong encounter with another star or internal planetary scattering, drive the evaporation independent of planet mass of orbital separation at birth. The effect of small perturbations due to slow changes in the cluster potential are important for the evolution of planetary systems. In addition, the probability of a star to lose a planet is independent of the planet mass and independent of its initial orbital separation. As a consequence, the mass distribution of free-floating planets indistinguishable from the mass distribution of planets bound to their host star.

• The paper models 500 stars with 2522 planets over 10 Myrs, showing that 357 planets become unbound due to gravitational interactions.
• The study finds that planet ejection is independent of planetary mass and orbital separation, resulting in a free-floating planet mass distribution similar to bound ones.
• The simulation reveals that early cluster dynamics drive planet ejections, providing key insights for observational strategies targeting free-floating planets.

Survivability of Planetary Systems in Young and Dense Star Clusters

The study under review presents a meticulous simulation of planetary systems within the venerable environment of the Orion Trapezium star cluster. This research leverages the Astrophysical Multipurpose Software Environment (AMUSE) to assess the survivability of planetary systems amidst the gravitational perturbations of densely packed, young star clusters. The primary focus is on the production of free-floating planets (FFPs), a significant aspect given the theoretical and observational indications of their abundance in the Milky Way.

Simulation Framework and Initial Conditions

The simulations accommodate 500 solar-like stars each hosting 4 to 6 planets, commencing from initial conditions that accurately reproduce the circumstellar disk properties observed in the Orion cluster. The planetary system configurations consider various masses and orbital separations up to 400 astronomical units. A comprehensive exploration into the dynamical outcomes of these systems is conducted over a simulated evolution of 10 million years.

Key Findings

1. Planet Ejection and Capture:
   • A total of 357 out of 2522 planets become unbound due to dynamical interactions, either through strong stellar encounters or internal planetary scattering. Among these, 281 leave the cluster entirely, whereas 76 prevail within the cluster as FFPs, with 5 being recaptured by other stars.
2. Independence from Planetary Properties:
   • The study reveals that the likelihood of planet ejection is notably independent of planetary mass and their initial orbital separation. Thus, the resulting mass distribution of FFPs parallels that of planets retaining their host stars.
3. Cluster’s Influence on Planetary Dynamics:
   • Perturbations induced by the slowly changing gravitational potential of the star cluster are integral to the observed dynamics. Such interactions underscore the cluster’s potent influence on the long-term evolution of planetary systems.
4. Cluster Dynamics and Free-Floating Planet Production:
   • Despite the dense cluster environment, the simulation results reinforce that encounters leading to planet ejection occur predominantly in the early stages of cluster evolution. This may elucidate the paucity of observed rogue planets in contemporary young clusters.

Implications and Future Directions

The simulation results proffer vital insights into the dynamics governing star clusters and the genesis of FFPs. For theoretical astrophysics, these findings enrich our understanding of planetary system evolution within complex gravitational milieus. Practically, the study informs observational strategies targeting FFP detection to align with mass distribution predictions and cluster origins.

Future work may further refine these models by incorporating more nuanced stellar evolution processes or extending simulations to different cluster morphologies. Additionally, elucidating the mechanisms behind those planets that avoid ejection remains a crucial endeavor. The exploration of wider separations or stronger magnetic interactions could provide additional insights. As observational techniques advance, validating these simulations against empirical data will prove essential in delineating the narrative of planetary systems and star clusters within our galaxy.