Around four billion years ago the Solar System may have been visited by a massive interloper whose passage disrupted the orbits of the outer giants. In a new line of inquiry, a team deployed thousands of computer simulations to explore how a passing body with different masses could perturb planetary orbits, testing a range of trajectories and speeds. The aim was to understand whether such an encounter could leave lasting marks on the arrangement of the planets we see today while acknowledging that many scenarios would create configurations far from the current layout. The work appeared as a preprint on arXiv, providing a detailed look at alternative histories for our planetary neighborhood.
A large set of simulations was run, totaling fifty thousand, to model the Solar System’s reaction to a passing object of varied mass. Each run treated gravitational interactions among the Sun, the four outer planets, and the intruder, tracking how their paths would evolve under a wide spectrum of encounter conditions. The goal was to map possible outcomes and identify the circumstances under which the giant planets might be nudged toward states resembling their familiar orbits.
Although the majority of the simulated encounters produced outcomes distinctly different from the Solar System we know, the researchers found that in roughly one percent of tests, the passing body altered the outer giants’ orbits in a way that was not far from today’s configuration. In those rare cases, the geometry and timing of the encounter aligned to produce a stable set of giant-planet orbits that echoed the current Solar System.
The closest scenario in the simulations involved a flyby by an object with a mass about eight times that of Jupiter, approaching the Sun at approximately 1.69 astronomical units. An astronomical unit is the average distance between the Earth and the Sun, about 149.6 million kilometers. In these terms, the interaction would have stretched over a scale similar to the orbit of Mars, situating the encounter at the inner edge of the outer planetary zone and illustrating the dramatic reach of such a flyby.
The inferred mass range of the intruding body spans from giant planets to brown dwarfs, objects that sit in the gray area between fully fledged planets and stars. This places the interloper squarely in a regime where gravity from a substellar mass can exert powerful, planet-scale perturbations without requiring stellar masses.
This type of event would have occurred in an era when star systems often interact in crowded stellar nurseries, years before the classical pathways of planet migration were fully established. The results demonstrate that external perturbations can reshape a planetary system by rearranging giant-planet orbits without necessarily ejecting every planet from the system, offering an alternative route to the architectures seen in the Solar System and beyond.
The findings contribute to a broader conversation about how external forces influence planetary formation and evolution. They provide a framework for interpreting unusual exoplanet configurations as possible evidence of past flybys, and they invite further study into how common such encounters might be in young star clusters. The work underscores that the Solar System’s quiet order could reflect a history shaped by rare yet influential external events.
Taken together, these simulations help illuminate the spectrum of possible planetary histories and remind scientists that the Solar System’s current arrangement is the product of a complex dance with passing strangers, one that helped shape the planets we observe today.