Captured Interstellar Objects Can Survive in Our Solar System for Billions of Years, Study Finds

Captured interstellar objects — rocky bodies gravitationally seized from the broader galaxy — survive in bound orbits around our Sun far longer than prior models suggested, according to a study published in The Planetary Science Journal in November 2021. Researchers Kevin J. Napier and Fred C. Adams of the University of Michigan, alongside Konstantin Batygin of the California Institute of Technology, ran 276,691 long-term N-body simulations to characterize the dynamical lifetime of alien rocks after capture — the most comprehensive treatment of this problem to date.

The work was prompted by the confirmed detections of 'Oumuamua (2017) and Borisov (2019), the first interstellar objects observed passing through the solar system. While neither was captured, both detections sharpened a fundamental question: does our solar system currently harbor material of extrasolar origin on stable, long-term orbits? Answering it required knowing not just how often capture occurs, but how long any captured object stays.

The team integrated the orbits of synthetic captured objects using the IAS15 integrator within the REBOUND N-body code, tracking each object for up to 1 billion years under gravitational perturbations from Jupiter, Saturn, Uranus, and Neptune. A simulation terminates when an object is ejected, collides with a body, or exceeds an apocenter distance of 50,000 au. The resulting dataset provides a statistically robust picture of how the captured population depletes over time.

A Power-Law Decay Governs Survival Over Gigayear Timescales

The surviving fraction f(t) — the probability that a captured object remains in a bound orbit at time t — follows a power-law decay after an initial plateau of a few million years. The best-fit analytical expression is f(t) = 1/(u² + 1), where u = t/τ, with a characteristic timescale τ ≈ 0.84 Myr and power-law index β ≈ 1.6. This form fits the numerical data across the entire range of tested lifetimes and is broadly consistent with predictions from a one-dimensional orbital energy diffusion model first described by Yabushita (1980).

A key physical implication is that the Lyapunov time of the outer solar system — the timescale over which orbital trajectories diverge chaotically — is roughly comparable to the transition timescale τ. Chaos governs ejection on short timescales, but the power-law tail extends survival to timescales orders of magnitude longer. Three orbital properties predict greater longevity: pericenters elevated beyond Jupiter's orbit, high inclinations relative to the ecliptic plane, and residence in the giant planet regime rather than the inner solar system.

The von Zeipel–Lidov–Kozai Resonance as a Survival Engine

The dominant mechanism sustaining long-lived captured objects is the von Zeipel–Lidov–Kozai (vZLK) effect — a secular gravitational resonance in which coupled oscillations of orbital inclination and pericenter distance allow a body to avoid planet-crossing trajectories for extended periods. Objects locked into vZLK cycles reach pericenter at argument of pericenter values near ω ≈ 90° or 270°, approaching the Sun from above or below the ecliptic, where encounters with the giant planets are strongly suppressed.

To validate this analytically, the team constructed a secular Hamiltonian framework, averaging over planetary perturbations to produce level curves in pericenter distance q versus argument of pericenter ω. For 11 of the 13 objects surviving longer than 500 Myr, the theoretical vZLK level curves matched the simulation data closely. All three retrograde survivors of the full 1 Gyr integration had inclinations near i ≈ 120°, directly consistent with the vZLK prediction.

Estimating the Current Inventory of Captured Interstellar Material

Combining the lifetime function with capture cross sections from their companion paper, the team calculated the present-day steady-state mass of interstellar rocky material in bound orbits. Objects captured continuously from the interstellar field contribute approximately 10⁻¹³ solar masses. The dominant contribution comes from material captured while the Sun resided in its birth cluster, when surrounding stellar density was orders of magnitude higher — leaving behind an estimated 10⁻⁹ solar masses of alien rocky material, roughly four orders of magnitude more than the field contributes today.

The same formalism was extended to captured dark matter. Dark matter particles in the Milky Way halo follow a Maxwellian velocity distribution with a dispersion of approximately 200 km/s — far above the solar escape velocity for most particles. Only the low-velocity tail of the distribution is gravitationally capturable. The resulting steady-state dark matter mass bound to the solar system is approximately 10¹⁷ grams, roughly 1,000 times less massive than Earth's atmosphere.

"The mass in alien rocks remaining from capture events in the solar birth cluster is much larger than the steady-state mass captured from the field. The estimated steady-state mass in dark matter particles has an intermediate value."

Limitations, Planet Nine, and the Path Forward

The authors identify several limitations. Only 13 objects survived beyond 500 Myr in the primary simulations, leaving the power-law tail of the lifetime function statistically sparse. The study also excluded galactic tides, passing stars, and the hypothesized Planet Nine from its primary runs. A supplemental set of approximately 40,000 integrations including a 10 Earth-mass body at a = 500 au produced orbital dynamics — including prograde-to-retrograde flips and pericenter lifts into the inner Oort Cloud — not seen in the four-planet case. The secular influence of Planet Nine on this population is identified as a priority for future investigation.

The research was supported by NASA's Planetary Science Program and the National Science Foundation under grant No. AST-2009096. As wide-field survey facilities including the Vera C. Rubin Observatory begin systematic operations, the dynamical lifetime function developed here provides a concrete theoretical framework for interpreting future detections of captured interstellar objects. The full study is available open access in The Planetary Science Journal at doi:10.3847/PSJ/ac29bb.