A close passage of a star billions of years ago may have affected the architecture of the Solar System, leading to the unusual orbits of trans-Neptunian objects and explaining the origin of the irregular moons of Jupiter and Saturn.

New data suggests that billions of years ago, a star may have passed very close to our solar system. As a result, thousands of small bodies in the outer Solar System beyond the orbit of Neptune were deflected into highly inclined orbits around the Sun. Perhaps some were captured as satellites by the planets Jupiter and Saturn.

When we think of our solar system, we usually assume that it ends at Neptune, the farthest known planet. “However, several thousand celestial bodies are known to extend beyond the orbit of Neptune,” explains astrophysicist Susanne Pfalzner from Forschungszentrum Jülich.

There are even suspicions that there are tens of thousands of objects with diameters exceeding 100 kilometers. “Surprisingly, many of these so-called trans-Neptunian objects move in eccentric orbits that are inclined with respect to the general plane of the orbits of the planets in the Solar System.”

Scientists from the Astronomy and Astrophysics Modelling and Data Laboratory at Forschungszentrum Jülich have conducted more than 3,000 computer simulations to find the reason for the unusual orbits of transneptational objects. The young sun is surrounded by a disk of material from which planets eventually form. There are several indications that a close pass by another star could have disrupted this disk. The simulations show how such a close flyby could have strongly affected the orbits of material in the outer disk. Many objects would be ejected into space on eccentric orbits. While the outer solar system would be completely rebuilt, the inner solar system, including our Earth, would remain intact. Credit: Forschungszentrum Jülich

Unraveling the trans-Neptunian mysteries

Susanne Pfalzner, together with her student Amit Govind and Simon Portegis Zwart from Leiden University, used more than 3,000 computer simulations to investigate the possible cause of the unusual orbits: Could the strange orbits of the trans-Neptunian objects be caused by another star?

Three astrophysicists have found that a significant close flyby of another star could explain the inclined and eccentric orbits of known trans-Neptunian objects. “Even very distant objects, such as the dwarf planet Sedna, discovered in 2003 in the farthest part of the Solar System, have orbits that can be determined. There are also objects that move in orbits that are almost perpendicular to the planets,” says Suzanne Pfalzner.

Such a close pass could even explain the orbits of 2008 KV42 and 2011 KT19, two celestial bodies that move in the opposite direction to the planets.

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“The best match for the modern exoplanet system that we found with our simulations is a star of about 0.8 solar masses, which is slightly lighter than our Sun,” explains Pfalzner’s colleague Amit Govind. “This star passed by our Sun at a distance of about 16.5 billion kilometers. That’s about 110 times the distance between Earth and the Sun, and a little less than four times the distance to the exoplanet Neptune.”

But the scientists’ most surprising finding was that the passing of an alien star billions of years ago could also provide a natural explanation for events happening nearby. Suzanne Pfalzner and her colleagues found in their simulations that some trans-Neptunian objects were hurled into our solar system – the region of the outer giant planets Jupiter, Saturn, Uranus and Neptune.

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“Some of these objects may have been captured as moons by giant planets,” says Simon Portegis Zwart of Leiden University. “This explains why the outer planets of our solar system have two different types of moons.”

Unlike regular satellites, which orbit close to the planet in circular orbits, irregular satellites orbit the planet at greater distances in inclined, elongated orbits. So far, this phenomenon has not been explained.

“The beauty of this model is its simplicity,” Pfalzner says. “It answers many open questions about our solar system for just one reason.”