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Implications for the formation of Oort cloud-like structures and interstellar comets in dense environments

Santiago Torres

Abstract

Most stars form in dense stellar environments, where frequent close encounters can strongly perturb and reshape the early architecture of planetary systems. The solar system, with its rich population of distant comets, provides a natural laboratory to study these processes. We perform detailed numerical simulations using the LonelyPlanets framework that combines NBODY6++GPU and REBOUND, to explore the evolution of debris disks around solar system analogues embedded in stellar clusters. Two initial configurations are considered, an $Extended$ and a $Compact$ model, each containing four giant planets and either an extended or compact debris disk. We find that compact disks primarily form Kuiper belt and scattered disk-like populations through planet-disk interactions, while extended disks are more strongly shaped by stellar encounters, producing Oort cloud-like structures and interstellar comets with ejection velocities of 1-3 km/s. Stellar perturbations are most effective for encounter inclinations between $0^{\circ}$ and $30^{\circ}$, giving rise to distinct dynamical populations, like Sednoids, and inner Oort cloud analogues, and a characteristic tail in semi-major axis-eccentricity space. In coplanar encounters, the disk remains largely flattened, whereas polar flybys redistribute angular momentum vertically, producing nearly isotropic outer populations that resemble an emerging Oort cloud. Our results suggest that cometary reservoirs and interstellar objects are natural byproducts of planet-disk interactions and stellar flybys in dense clusters, linking the architecture of outer planetary systems to their birth environments.

Implications for the formation of Oort cloud-like structures and interstellar comets in dense environments

Abstract

Most stars form in dense stellar environments, where frequent close encounters can strongly perturb and reshape the early architecture of planetary systems. The solar system, with its rich population of distant comets, provides a natural laboratory to study these processes. We perform detailed numerical simulations using the LonelyPlanets framework that combines NBODY6++GPU and REBOUND, to explore the evolution of debris disks around solar system analogues embedded in stellar clusters. Two initial configurations are considered, an and a model, each containing four giant planets and either an extended or compact debris disk. We find that compact disks primarily form Kuiper belt and scattered disk-like populations through planet-disk interactions, while extended disks are more strongly shaped by stellar encounters, producing Oort cloud-like structures and interstellar comets with ejection velocities of 1-3 km/s. Stellar perturbations are most effective for encounter inclinations between and , giving rise to distinct dynamical populations, like Sednoids, and inner Oort cloud analogues, and a characteristic tail in semi-major axis-eccentricity space. In coplanar encounters, the disk remains largely flattened, whereas polar flybys redistribute angular momentum vertically, producing nearly isotropic outer populations that resemble an emerging Oort cloud. Our results suggest that cometary reservoirs and interstellar objects are natural byproducts of planet-disk interactions and stellar flybys in dense clusters, linking the architecture of outer planetary systems to their birth environments.

Paper Structure

This paper contains 18 sections, 8 equations, 15 figures, 1 table.

Table of Contents

  1. Introduction
  2. Numerical Implementation
  3. Single Stellar Encounter
  4. Multiple Stellar Encounters
  5. Dynamical evolution of a debris disk in dense environments
  6. Extended models
  7. Extended 1
  8. Extended N
  9. Compact models
  10. Compact 1
  11. Compact N
  12. Extended vs Compact models
  13. Interstellar comets
  14. Interstellar comets from single close encounters
  15. Summary and Conclusions
  16. ...and 3 more sections

Figures (15)

  • Figure 1: Extended 1 model. Semi-major axis as a function of eccentricity and particle count after an encounter with a $1$$M_\odot$ star at 300 au. The colour bar indicates the perihelion distance of the particles. Each panel corresponds to a different encounter inclination angle ($0^{\circ}$, $30^{\circ}$, $60^{\circ}$, and $90^{\circ}$).
  • Figure 2: Extended 1 model. Cumulative distribution of the final energy of particles in the disk. Coloured lines correspond to different inclination angles of the encounter ($0^{\circ}$, $30^{\circ}$, $60^{\circ}$, and $90^{\circ}$), while the black dotted line represents the initial particle distribution as defined in Table \ref{['table1']}. Negative energy values indicate particles that remain bound to the system after the encounter, whereas positive values correspond to interstellar objects. The Kolmogorov-Smirnov (KS) probabilities relative to the initial distribution for the different angles are $0.90$% for $0^{\circ}$, $0.19$% for $30^{\circ}$, $0.07$% for $60^{\circ}$, and $0.05$% for $90^{\circ}$.
  • Figure 3: Extended N model. Semi-major axis as a function of eccentricity (top panel) and orbital inclination (bottom panel) for all the particles in the 200 simulated systems. The grey areas represent the different regions of the solar system (Kuiper belt (KB), scattered disk (SD), Sednoids, inner and outer Oort cloud (OC)), while the dashed lines show the 2:3, 1:2, and 2:5 resonances with Neptune. The integration time is set to 100 Myr. An animation can be found online.
  • Figure 4: Extended N model. Orbital evolution of particles in the disk over 100 Myr. The bottom panels in each plot depict the semi-major axis as a function of time, while the top panels show the distance of the perturber. Each panel represents the systems: 128, 14, 100, 86, and 157. Coloured lines correspond to individual particles within each system.
  • Figure 5: Extended N model. Orbital elements of particles in the disk after 100 Myr and multiple stellar encounters. The first column presents the semi-major axis as a function of eccentricity, with particles colour-coded by their perihelion distances. The green, blue, and grey shaded regions highlight different populations formed due to stellar encounters. The second column displays perihelion as a function of orbital inclination, with dots colour-coded by aphelion distance. The grey shaded areas represent distinct regions of the solar system, including the Kuiper Belt (KB), scattered disk (SD), Sednoids, and the inner Oort Cloud (OC). The third column illustrates the distribution of orbital energy for the particles, where red histograms correspond to the initial energy distribution, and the blue curve represents the final energy distribution. Blue curves with positive values indicate interstellar comets. Each row corresponds to systems numbered 128, 14, 100, 86, and 157, respectively.
  • ...and 10 more figures