Solution to a cosmic mystery

Flyby of a star explains dynamics of the solar system beyond Neptune.

When we think of our solar system, we usually assume that it ends at the outermost known planet, Neptune. „However, several thousand celestial bodies are known to move beyond the orbit of Neptune,“ explains Susanne Pfalzner, astrophysicist at Forschungszentrum Jülich. It is even suspected that there are tens of thousands of objects with a diameter of more than 100 kilometres. „Surprisingly, many of these so-called trans-Neptunian objects move on eccentric orbits that are inclined relative to the common orbital plane of the planets in the solar system.“

Together with her student Amith Govind and Simon Portegies Zwart from Leiden University, Susanne Pfalzner has used more than 3,000 computer simulations to investigate a possible cause of the unusual orbits: could another star have caused the strange orbits of trans-Neptunian objects?

The three astrophysicists found that a distinctive, close flyby of another star can explain the inclined and eccentric orbits of the known transneptunian celestial bodies. „Even the orbits of very distant objects can be deduced, such as that of the dwarf planet Sedna in the outermost reaches of the solar system, which was discovered in 2003. And also objects that move in orbits almost perpendicular to the planetary orbits,“ says Susanne Pfalzner. Such a flyby can even explain the orbits of 2008 KV42 and 2011 KT19 – the two celestial bodies that move in the opposite direction to the planets.

„The best match for today’s outer solar system that we found with our simulations is a star that was slightly lighter than our Sun – about 0.8 solar masses,“ explains Pfalzner’s colleague Amith Govind. „This star flew past our sun at a distance of around 16.5 billion kilometres. That’s about 110 times the distance between Earth and the sun, a little less than four times the distance of the outermost planet Neptune.“

However, the scientists’ most surprising realisation was that the flyby of an alien star billions of years ago could also provide a natural explanation for phenomena closer to home. Susanne Pfalzner and her colleagues found that in their simulations, some trans-Neptunian objects were hurled into our solar system – into the region of the outer giant planets Jupiter, Saturn, Uranus and Neptune.

“Some of these objects could have been captured by the giant planets as moons,” says Simon Portegies Zwart from Leiden University. “This would explain why the outer planets of our solar system have two different types of moons.” In contrast to the regular moons, which orbit close to the planet on circular orbits, the irregular moons orbit the planet at a greater distance on inclined, elongated orbits. Until now, there was no explanation for this phenomenon. “The beauty of this model lies in its simplicity,” says Pfalzner. “It answers several open questions about our solar system with just a single cause.”

Original publications:

“Trajectory of the Stellar Flyby Shaping the Outer Solar System”
by Susanne Pfalzner, Amith Govind & Simon Portegies Zwart, Nature Astronomy (2024)
DOI: 10.1038/s41550-024-02349-x

“Irregular moons possibly injected from the outer solar system by a stellar flyby”,
by Susanne Pfalzner, Amith Govind, and Frank W. Wagner, September 4 2024.
The Astrophysical Journal Letters, Volume 972, Number 2
DOI: 10.3847/2041-8213/ad63a6

Journal: Nature Astronomy
DOI: 10.1038/s41550-024-02349-x
Method of Research: Computational simulation/modeling
Subject of Research: Not applicable
Article Title: Trajectory of the stellar flyby that shaped the outer Solar System
Article Publication Date: 4-Sep-2024