The ejection velocities of interstellar objects signpost their progenitor system architectures

TL;DR

This paper investigates how planetary-system architectures shape the production and kinematics of interstellar objects (ISOs) ejected after gas-d disc dispersal. Using 2,461 high-mass, multi-planet system simulations, each with a debris disc of 10,000 test particles and integrated for 10 Myr, the authors identify two main ISO ejection pathways: a quiet mode with modest ejections and lower $v_\infty$, and a catastrophic mode with extensive ejections and higher $v_\infty$. They show a robust bimodality in the $v_\infty$–$f_\mathrm{ej}$ plane, further decomposed into sub-clusters that map to distinct architectural traits (e.g., monarchic versus cascading instabilities). The findings imply that ISO velocity distributions encode the assembly history of their birth systems and can inform Galactic archaeology and population modeling, with velocities typically of a few km s$^{-1}$ and slow ejections spelling out dense Galactic tidal streams.

Abstract

Interstellar objects (ISOs) ejected from planetary systems carry kinematic signatures of their formation environments. The properties of these velocity distributions govern the ISOs' propagation and dynamical evolution in the Galactic potential. We investigate how planetary system architecture influences ISO production during post-gas-disc dynamical instabilities using N-body simulations. We explore the ISO production outcomes of 2461 randomly generated systems spanning total system masses of 300-800 Earth masses and multiplicities of 3-7 planets. By integrating planets embedded in a disc of test particles for 10 Myr, we find that evolving systems can be broadly divided into two distinct classes based on their initial architectures. Catastrophic systems are characterized by high multiplicities and orbitally compact configurations, or by high-mass planets in systems with large mass asymmetries. These systems eject a large fraction of their planetesimals (median 59 percent) and, depending on the ejection pathway, produce high-speed ISOs (median 2.9 km/s). In contrast, quiet systems have lower masses and multiplicities and do not undergo significant orbital rearrangement, yet still eject a median of 28 percent of planetesimals at lower velocities (median 1.6 km/s). This dichotomy points to distinct ejection pathways, involving either violent global instabilities or more gradual, diffusive processes. Overall, we find that ISO ejection velocities are typically low, on the order of a few km/s. Although ISOs subsequently experience dynamical heating as they orbit the Galaxy, their velocity distributions retain signatures of their progenitor systems' architectures and histories, underscoring the potential use of ISOs in Galactic archaeology.

Figures (16)

• Figure 1: Left: Safronov number for the population of known exoplanets, following Fig. 1 of laughlin2017. Only planets with measured masses and radii are shown (data from the NASA Exoplanet Archive, 22 Nov 2025). Solar System planets are plotted as larger symbols with black outlines. The pink line indicates a radial-velocity semi-amplitude precision of 1 m s$^{-1}$lovis2010, indicating the approximate detection threshold for RV surveys. The grey shaded region marks the Kepler primary mission duration, serving as a proxy for transit-survey sensitivity. Right: distribution of $\Theta$ for the exoplanets in the left panel. Most known exoplanets have $\Theta \ll 1$, implying that their dynamical interactions are collision-dominated rather than ejection-dominated.
• Figure 2: Thirty of the generated initial system configurations. Colour encodes planetary mass, indicating approximate compositional categories. Red planets are larger than 80 M$_\oplus$ (gas giants), and blue planets are from 5-80 M$_\oplus$ (gas and ice giants). The green planets are below 5 M$_\oplus$, indicating terrestrial planets. Symbol sizes are proportional to planetary radii calculated from \ref{['eq:massradius']}.
• Figure 3: Semi-major axis time series for planets and 16 randomly chosen ejected planetesimals in the example system. Colours from blue to green indicate ejection velocity magnitude for ISOs. The red curve shows the ejected 10 M$_\oplus$ planet; black curves show planets that remain bound at 10 Myr.
• Figure 4: Ejection time, perihelion distance, and velocity for all ISOs in the example system. Early ejections ($t < 2$ Myr) from inner regions show the highest velocities. Ejection velocity decreases with perihelion distance. The grey histogram shows the ejection flux, which peaks at 2 Myr after simulation initialisation, and then steadily decreases.
• Figure 5: Temporal evolution of ISO ejection velocities in the Solar System simulation of nesvorny2023. Each point shows an ejected particle’s $v_\infty$ at the time of ejection, calculated from its orbital elements at 500,000 au and traced back to perihelion passage time. Colour indicates perihelion distance, while vertical dashed lines correspond to strong stellar flyby events that perturb the system. Additional vertical features may indicate further stellar encounters.