Search for Past Stellar Encounters and the Origin of 3I/ATLAS (2509.03361v1)

Abstract: 3I/ATLAS, the third discovered interstellar object, has a heliocentric speed of 58 km/s and exhibits cometary activity. To constrain the origin of 3I/ATLAS and its past dynamical evolution, we propagate the orbits of 3I/ATLAS and nearby stars to search for stellar encounters. Integrating orbits in the Galactic potential and propagating the astrometric and radial-velocity uncertainties of 30 million Gaia stars, we identify 25 encounters with median encounter distances less than 1 pc. However, because the encounter speeds between 3I/ATLAS and each encounter exceed 20 km/s, none is a plausible host under common ejection mechanisms. We infer stellar masses for most stars and quantify the gravitational perturbations exerted by each individual star or each binary system on 3I/ATLAS. The strongest gravitational scattering perturber is a wide M-dwarf binary. Among all past encounters, the binary's barycenter and 3I/ATLAS reach the small encounter distance of 0.242 pc and the encounter speed of 28.39 km/s,1.64 Myr ago. We further demonstrate that the cumulative influence of the stellar encounters on both the speed and direction of 3I/ATLAS is weak. Based on the present kinematics of 3I/ATLAS to assess its origin, we find that a thin-disk origin is strongly favored, because the thin disk both exhibits a velocity distribution closely matching that of 3I/ATLAS and provides the dominant local number density of stars.

• The paper demonstrates that high-precision Gaia DR3 astrometry combined with Monte Carlo orbital integration effectively constrains past stellar encounters with 3I/ATLAS.
• It identifies 25 close encounters, notably a wide M-dwarf binary with high encounter speed, which is inconsistent with typical ejection mechanisms.
• Kinematic analysis yields a 96.6% probability for a thin-disk origin, confirming that stellar scattering has a negligible impact on 3I/ATLAS's high velocity.

Stellar Encounter Constraints on the Origin of 3I/ATLAS

Introduction

The discovery of 3I/ATLAS, the third confirmed interstellar object (ISO) traversing the Solar System, provides a unique opportunity to probe the dynamical and physical processes governing the ejection and propagation of small bodies in the Galaxy. This paper presents a comprehensive search for past stellar encounters of 3I/ATLAS using Gaia DR3 astrometry and radial velocities, with the aim of constraining its origin and evaluating the plausibility of host-star associations. The analysis leverages high-precision orbital integration, Monte Carlo uncertainty propagation, and kinematic population modeling to assess the dynamical history and likely source population of 3I/ATLAS.

Data Selection and Orbital Integration

The analysis utilizes a subset of approximately 30 million Gaia DR3 stars with high-quality astrometry and radial velocities, filtered for fractional parallax errors below 20%. The initial conditions for both stars and 3I/ATLAS are transformed into the galactocentric frame, with the latter's state vector referenced to J2016.0. The orbital integration is performed in a Milky Way potential constrained by recent rotation curve measurements, using the galpy package with a 2000-year timestep over a ±100 Myr window.

A two-stage encounter search is implemented: a linear approximation pre-filters candidates with nominal encounter distances below 50 pc, followed by full nonlinear integration for those within 10 pc. Monte Carlo sampling of astrometric and kinematic uncertainties is performed for both stars and 3I/ATLAS, yielding 200,000 realizations per candidate. The final selection comprises 25 stars (including binary barycenters) with median encounter distances below 1 pc.

Figure 1: Distributions of nominal and median encounter distances $d^{\mathrm{enc}}$ and encounter speeds $v^{\mathrm{enc}}$ for close encounters, with error bars indicating 5% and 95% quantiles from Monte Carlo results.

Encounter Uncertainties and Dominant Error Sources

The propagation of uncertainties is dominated by the stellar astrometric and radial velocity errors, rather than the uncertainties in the 3I/ATLAS state vector. This is particularly evident for encounters at large lookback times, where the dispersion in encounter parameters is set by the stellar measurement precision.

Figure 2: Monte Carlo comparison of encounter uncertainties, highlighting the dominant contribution from stellar parameter errors over those of 3I/ATLAS.

Analysis of Individual Encounters

Among the 25 close encounters, the strongest gravitational perturber is a wide M-dwarf binary (G 137-55/G 137-54), with a barycentric median encounter distance of $0.242^{+0.089}_{-0.084}$ pc and a median encounter speed of $28.39^{+0.67}_{-0.67}$ km/s, occurring 1.64 Myr ago. The perturbation significance, quantified by the $g$ parameter, is an order of magnitude higher for this system than for any other candidate. However, the encounter speed is well above the typical ejection velocities produced by known binary or planetary scattering mechanisms, rendering a direct host-star association implausible.

Figure 3: Distributions of deflection angle $\delta$ and perturbation proxy $g$ for the strongest binary encounter, smoothed with a Gaussian kernel.

Other notable encounters include nearby M-dwarf binaries (e.g., V* AT Mic A/B), which, despite their youth and planetesimal-rich environments, have kinematics inconsistent with the high LSR-relative velocity of 3I/ATLAS. Encounters with K- and G-type stars are also excluded as plausible sources due to high relative velocities and insufficient gravitational influence at the encounter distances.

Cumulative Effects of Stellar Encounters

The cumulative dynamical effect of all encounters within 1 pc over Gyr timescales is evaluated using analytic impulse approximations and Monte Carlo simulations. The total speed change imparted to 3I/ATLAS is found to be negligible, with only minor deflections in trajectory direction. This result is robust even when assuming all encountering stars have solar mass, which overestimates the true effect.

Figure 4: Histogram of relative speeds between 3I/ATLAS and $\sim$134,000 nearby stars, illustrating the high-velocity tail occupied by 3I/ATLAS.

Figure 5: Cumulative speed change $\Delta v$ and deflection angle $\delta$ from all encounters, demonstrating the weak net dynamical effect on 3I/ATLAS.

These findings imply that 3I/ATLAS must have acquired its high LSR-relative velocity at the time of ejection, rather than through subsequent stellar scattering.

Kinematic Population Analysis and Origin Probability

The present-day velocity of 3I/ATLAS is compared to the empirically determined velocity ellipsoids of the Galactic thin and thick disks. The likelihood ratio for a thin-disk versus thick-disk origin, based on three-dimensional velocity and local population fractions, yields a posterior thin-disk probability of 96.6% (for an assumed age of 5 Gyr), with only weak dependence on age over the range 3–15 Gyr. Restricting the analysis to the vertical velocity component alone reduces the discriminating power, but the inclusion of population priors still favors a thin-disk origin.

This result is in tension with previous studies that suggested a thick-disk origin based on maximum vertical excursion, but the present analysis demonstrates that $z_{max}$ is insufficient to distinguish disk membership in this regime.

Implications and Future Directions

The absence of plausible host-star associations for 3I/ATLAS, despite exhaustive search and high-precision astrometry, underscores the challenge of tracing ISOs to their source systems, particularly for objects with high ejection velocities. The strong kinematic evidence for a thin-disk origin, combined with the dominance of thin-disk stars in the local volume, suggests that the majority of ISOs detected in the Solar System will be drawn from this population.

The paper highlights the need for improved stellar parameterization (e.g., ages, masses, binarity) in future Gaia releases and complementary surveys to refine encounter analyses. As the sample of ISOs grows, statistical approaches to population inference and host-star association will become increasingly important for constraining the dynamical and compositional diversity of extrasolar small bodies.

Conclusion

This work provides a rigorous dynamical and kinematic analysis of the past stellar encounters of 3I/ATLAS, demonstrating that none of the 25 closest encounters identified in Gaia DR3 are plausible host stars due to high relative velocities. The cumulative effect of stellar scattering is shown to be dynamically insignificant for the current velocity of 3I/ATLAS. The kinematic properties of 3I/ATLAS are overwhelmingly consistent with a thin-disk origin, with a posterior probability exceeding 96%. These results reinforce the view that ISOs detected in the Solar System are most likely to originate from the Galactic thin disk, and that direct host-star associations will remain elusive for high-velocity objects. Future advances in astrometric precision, stellar characterization, and ISO discovery rates will be essential for further progress in this domain.