The ejection velocities of interstellar objects signpost their progenitor system architectures

Interstellar objects (ISOs) are released from their progenitor planetary systems through dynamical mechanisms that impart a wide range of ejection velocities. These velocity distributions influence the propagation and long-term dynamical evolution of ISOs within the Galactic potential. Understanding these distributions is critical for interpreting the population of ISOs that will be observed in Rubin’s Legacy Survey of Space and Time (LSST) and by NEOSurveyor.

We assess the effect of dynamical instabilities across a broad range of planetary system architectures on both the ejection rates and velocities of ISOs. Specifically, we consider gravitational scattering driven by large-scale dynamical instabilities—a common evolutionary pathway for planetary systems. We performed an ensemble of more than 2,000 N-body simulations using the REBOUND package, exploring varying numbers of planets and total planetary system masses embedded within a massless planetesimal disk. We tracked planetesimal ejections over the first 10 Myr of each system’s evolution, following gas disk dispersal.

1: Thirty example systems in their initial configuration.

We find that ejection velocities are generally low, with typical values of only a few km/s for systems undergoing instability-driven scattering. The ejection velocity and the fraction of planetesimals ejected are strongly correlated, allowing us to identify two primary categories of planetary systems. In the first, ejection velocities cluster around 1–2 km/s, with ejection fractions ranging from 0.1 to 0.8. In the second, ejection velocities span 2–5 km/s, and the corresponding ejection fractions are consistently above 0.4. These categories are distinguishable based on system architecture parameters: lower ejection fractions are associated with high mass partitioning, low planetary multiplicity, overall lower total system mass, and the ejection of fewer planets during the simulation—indicative of the absence of a global dynamical instability.

While ISOs undergo subsequent dynamical heating as they orbit through the Galaxy, our results suggest that their initial kinematic distributions retain a memory of their parent systems’ architectures and dynamical histories. This influence may be observable in the present-day Galactic ISO population. It offers a new avenue for constraining the dynamical histories of exoplanetary systems, through ISO surveys.

2: Scatter plot of all of the completed integrations, showing the two clustered ejection fraction and velocity trends.