Quasi-interstellar objects: when the Solar System meets its own exiles
The possibility of return of small bodies ejected from the solar system due to erosion of the outer Oort cloud over the last few hundred million years (excluding the last 10 million years) has been investigated. It is shown that some of these objects, after moving in the galactic potential near the Sun but outside the tidal radius, can re-approach the solar system. Bodies that left the system at earlier stages are too scattered across the galaxy and do not contribute noticeably. These 'quasi-interstellar objects' (quasi-ISOs) are predicted to be very rare and have a velocity at infinity on the order of 0.1 km/s, sharply distinguishing them from objects of stellar origin. Their detection would indicate anomalously high losses from the Oort cloud or a catastrophic erosion event 10–300 million years ago, not detectable by other means.
Context
Since the discovery of comet 1I/‘Oumuamua, the first confirmed interstellar object, scientists have wondered: could some of these bodies actually originate from our own Oort Cloud? Although 1I/‘Oumuamua’s orbit was solidly hyperbolic and unbound from the Solar System, in principle comets ejected from the Oort Cloud by the tidal forces of the Galaxy and stellar close encounters could, after a long journey, meet the Sun again. The catch is that these “quasi-interstellar objects” could skew the statistics of genuine interstellar visitors if we can’t tell them apart. Understanding this phenomenon requires not only the dynamics of ejection but also the long-term orbital evolution in the Milky Way’s potential, which, as Vera Rubin and Fritz Zwicky showed, is largely shaped by dark matter.
Methods
To estimate the return probability, researchers simulated the ejection of “packets” of particles at low speeds (around 0.1 km/s) from a sphere of radius 1.5 parsecs around the Sun in the past, at intervals from 1 to 800 million years ago. Each particle’s motion, along with the Sun’s, was tracked in the smooth potential of the Galaxy, including the contribution from the dark halo, with an added stochastic “heating”—either white noise or correlated perturbations mimicking encounters with giant molecular clouds. Then, for each simulation, the probability density in phase space was computed, and an adaptive kernel estimate was used to determine the flux of returning particles through a sphere of 0.1 pc radius, limited by the condition of a pericenter less than 5 AU. Three models of Oort Cloud erosion were employed in parallel: simple exponential losses, data from N-body simulations, and a Monte Carlo method to account for stellar flybys—all to weigh the contributions from different ejection epochs.
Results
It turned out that the return probability for a single random particle is vanishingly small—no more than a few times 10^{-14} per year. The main contribution to the quasi-interstellar population comes from comets lost by the Oort Cloud over the last 500 million years, and those ejected around 60 million years ago are the most efficient at coming back. Their hyperbolic excess speeds are almost always below 1 km/s, with typical values around 0.1 km/s—hundreds of times slower than known interstellar objects. The radiants (incoming directions) on the celestial sphere cluster near the plane of the Galaxy at longitudes ~45° and ~225°, rather than toward the Sun's motion apex, where genuine interstellar objects predominantly aim. The absolute occurrence rate of quasi-interstellar bodies strongly depends on normalization: under the optimistic assumption that the Oort Cloud originally contained around 5×10^{14} 1I-sized objects, one might expect about two close passages per decade within 5 AU. But more conservative estimates drop that number to one event every few decades or even centuries. In any case, quasi-interstellar objects turn out to be rare and easily distinguishable by their kinematics.
Implications
These results mean that quasi-interstellar bodies will not create a significant “foreground” for future interstellar object surveys, since their low speeds and specific radiants immediately betray their origin. On the other hand, if such an object is ever detected—say, with JWST or the LSST survey—it would point to unusually intense erosion of the Oort Cloud in the past, possibly triggered by a close stellar flyby 10–300 million years ago. That would place a unique constraint on the dynamical history of the Solar System, one inaccessible by other methods. Moreover, studying the composition of returning comets through spectroscopy could shed light on the amounts of carbon and water ice in primordial planetesimals, as well as on the differences between our system and exoplanetary systems.
Future development
Further progress in understanding the quasi-interstellar population will come from refining the potential of the Galaxy, including the contribution of dark matter, and from building more detailed Oort Cloud erosion models using real stellar encounter data from the Gaia mission. New sky surveys, such as LSST, will significantly expand the statistics on long-period comets and interstellar bodies, allowing us to test predictions about the frequency and incoming directions of quasi-ISOs. It would also be fascinating to explore the possibility of capture of such objects by giant planets and their subsequent evolution.
Impact
This work spans several fields at once: the dynamics of small bodies in the Solar System, modeling of the galactic potential and dark matter, and the interpretation of data from future surveys, including spectroscopic studies of interstellar object composition with JWST.
Next steps
Next steps include running more detailed simulations that account for the chemical composition of ejected bodies and their possible fragmentation, as well as incorporating real stellar trajectories from Gaia catalogs into the models to accurately estimate the rate of disruptive close encounters.
Key open problems
The quasi-interstellar object problem is closely tied to uncertainties in the initial mass function of planetesimals and the Oort Cloud’s loss history—key issues in the formation and evolution of the Solar System. It also resonates with the challenge of mapping the distribution of dark matter in the local Galaxy, since the gravitational potential governs the scattering of stellar streams.
🎯 The fastest known interstellar object, 1I/‘Oumuamua, had an excess speed of about 26 km/s—260 times faster than the predicted speed of quasi-interstellar bodies. If such a sluggish object entered the Solar System, its orbit would be almost impossible to distinguish from those of ordinary Oort Cloud comets.
Key numbers
- typical excess speed of quasi-ISOs: ~0.1 km/s
- return probability per comet per year: less than 3×10^{-14}
- predicted events per decade (optimistic): ~2 within 5 AU
- number of Oort Cloud objects >100 m: ~5×10^{12}
- typical speed of genuine interstellar objects: ~60 km/s