Oort cloud (exo)planets (2306.11109v1)

Abstract: Dynamical instabilities among giant planets are thought to be nearly ubiquitous, and culminate in the ejection of one or more planets into interstellar space. Here we perform N-body simulations of dynamical instabilities while accounting for torques from the galactic tidal field. We find that a fraction of planets that would otherwise have been ejected are instead trapped on very wide orbits analogous to those of Oort cloud comets. The fraction of ejected planets that are trapped ranges from 1-10%, depending on the initial planetary mass distribution. The local galactic density has a modest effect on the trapping efficiency and the orbital radii of trapped planets. The majority of Oort cloud planets survive for Gyr timescales. Taking into account the demographics of exoplanets, we estimate that one in every 200-3000 stars could host an Oort cloud planet. This value is likely an overestimate, as we do not account for instabilities that take place at early enough times to be affected by their host stars' birth cluster, or planet stripping from passing stars. If the Solar System's dynamical instability happened after birth cluster dissolution, there is a ~7% chance that an ice giant was captured in the Sun's Oort cloud.

• The paper demonstrates that galactic tides capture 1-10% of ejected planets into wide, stable orbits analogous to an Oort cloud.
• Methodology employs N-body simulations to reveal that once captured, most planets endure on billion-year timescales.
• Implications include revised occurrence rate estimates and guidance for future detection of faint, distant exoplanetary bodies.

Oort Cloud (Exo)Planets: Dynamics and Implications

The paper on Oort cloud (exo)planets presented by Raymond, Izidoro, and Kaib explores the role of dynamical instabilities in giant planet systems and how such processes can lead to the formation of planets analogous to those found within the Solar System's Oort cloud. This work employs N-body simulations to explore the fates of planets during dynamical instabilities while taking into account the effects of galactic tides—a factor previously given limited attention in similar investigations. The findings suggest intriguing possibilities for the structure and composition of planetary systems, both within and beyond our own.

Summary of Key Results

1. Planetary Ejection and Trapping: The simulations indicate that a notable fraction (1-10%) of planets that would typically be ejected into interstellar space due to dynamical instabilities are instead caught in wide, stable orbits reminiscent of Oort cloud comets. This conversion from ejection to stable orbit is attributed to torques exerted by the galactic tidal field.
2. Survival on Gyr Timescales: Once captured, the majority of these Oort cloud planets survive on billion-year timescales. This demonstrates the long-term stability of such planets once their perihelia have been sufficiently lifted by galactic torques, effectively decoupling them from interactions with the inner planetary system.
3. Occurrence Rate Estimates: Based on exoplanet demographics, the authors estimate that approximately one in 200 to 3000 stars might host an Oort cloud planet. This estimation highlights that such bodies could be more common than might be expected, although the number is potentially an overestimation due to unmodeled early-stage instabilities.
4. Role of the Galactic Environment: The local density of the galactic environment has a modest impact on trapping efficiency and determines the orbital radii of trapped planets. High-density regions potentially enhance trapping by allowing galactic tides to exert stronger influence, although the long-term stability may be compromised by more frequent stellar encounters.

Theoretical Implications

The research reinforces the 'planet-planet scattering' model as a robust explanation for the eccentricity distributions observed in giant exoplanets. It offers a framework for understanding another consequence of these scattering events—capturing ejected planets into Oort cloud-like orbits. This perspective can profoundly affect our theoretical understanding of planetary system evolution, particularly in dynamically active systems.

Furthermore, the paper suggests that the mass hierarchy within a planetary system significantly influences the outcomes of dynamical instabilities. Lower-mass planets are more prone to ejection and capture, indicating that exoplanetary Oort clouds might primarily comprise sub-Jovian mass planets.

Practical Implications and Future Directions

Practically, the existence of Oort cloud exoplanets could influence future exoplanet detection strategies, particularly those that aim to identify faint, distant bodies in wide orbits. Observing such planets could provide insights into the early dynamical history of planetary systems.

Looking ahead, the paper hints at several avenues for future research:

• Investigating the frequency and characteristic properties of Oort cloud planets in different galactic environments, akin to exploring various parts of the Milky Way.
• Exploring the interaction of early planetary instabilities within stellar birth clusters.
• Delving into the role of lower-mass bodies as potential Oort cloud residents. This could significantly adjust our understanding of Oort cloud occurrence rates, necessitating detailed modeling of planetesimal ejection and capture dynamics.

In summary, the investigation of Oort cloud (exo)planets opens new horizons in exoplanet research, providing a distinctive angle to probe the vast and varied architecture of planetary systems across our galaxy.