When Stars Collide: Evidence for a Stellar Flyby in the Solar System’s Past
The Solar System’s trans-Neptunian objects (TNOs), icy bodies orbiting beyond Neptune, exhibit an unexpected pattern: their surface colors, ranging from very red to grey, correlate with their orbital dynamics rather than simply their distance from the Sun. Red TNOs dominate the “cold” Kuiper belt with low-inclination, nearly circular orbits, while grey TNOs are common in the “hot” population with more inclined and eccentric paths. Susanne Pfalzner and colleagues explore whether this puzzling connection could be explained by a close encounter with another star early in the Solar System’s history. They model a stellar flyby involving a star about 80% the Sun’s mass, passing within 110 astronomical units at a steep angle, which seems to reproduce both the observed orbital and color patterns.
Method
The team simulated a disk of 10,000–50,000 massless particles extending up to 150 AU from the Sun, representing the primordial TNO population. Initially, the disk had a smooth color gradient: the inner cold Kuiper belt was populated by very red bodies, while the outer regions beyond ~50 AU contained greyer material. When the simulated star passed by, it disrupted the disk, creating spiral arms that flung some material inward and some outward, mixing orbits and redistributing colors. Using advanced N-body simulation codes, the authors tracked the evolution of this disturbed disk over a billion years to assess how well it matched current observations.
Results
The simulations showed that a stellar flyby naturally generates the observed TNO patterns: most material stays bound to the Sun but is redistributed into inclined and eccentric orbits, forming the familiar “hot” and “cold” populations. Spiral arms emerge during the encounter, pushing grey material from the outer disk into higher-inclination orbits, while red material remains concentrated at low inclinations and eccentricities. The simulated TNOs’ color and dynamical distributions closely resemble survey data, suggesting that the flyby hypothesis is a viable explanation.
Inclination and Eccentricity Correlations
Comparing simulations with observational surveys, the team finds that very red TNOs are concentrated at low inclinations (below about 21°) and low eccentricities (below about 0.42), just as observed. At higher inclinations and eccentricities, grey TNOs dominate, originating from the outer, greyer parts of the disk. The scarcity of red TNOs among “detached” TNOs (those beyond Neptune’s strong influence) also fits this picture, as these bodies come from distant regions disturbed by the flyby.
Long-term Evolution and Predictions
After a billion years of evolution, the TNO patterns produced by the flyby remain recognizable, though somewhat smoothed out. The authors predict that upcoming surveys with the Vera Rubin Observatory will find that most distant TNOs beyond 60 AU are grey, and that retrograde TNOs (orbiting opposite the planets) will show a unique combination of light red and blue-grey colors. Such findings would provide strong evidence in favor of the flyby scenario.
Discussion
The study notes that observational biases--like the difficulty of detecting faint, distant TNOs--could influence our view of the population, but these biases likely do not account for the strong inclination–color and eccentricity–color correlations observed. The authors compare their results with other models, such as planetary migration theories, but those models struggle to explain the full set of observations, particularly for retrograde and detached objects. The flyby model also provides a natural explanation for the colors of the irregular moons of the giant planets, which lack very red bodies and may have been captured from the disturbed outer disk.
Conclusion
Pfalzner and colleagues conclude that a stellar flyby offers a simple and powerful explanation for the observed structure and color distribution of the TNO population. The encounter would have stirred grey material from the outer disk into excited orbits while leaving the redder inner disk more intact. They predict that future observations will test this idea by revealing more grey TNOs at large distances and distinct color patterns among retrograde objects. If these predictions hold, they would confirm that a close stellar flyby helped shape the outer Solar System.
Source: Pfalzner
