At least 140 million Sun-like stars in our Milky Way Galaxy are likely to have experienced a similar stellar flyby, according to new research by a team of astrophysicists from the Forschungszentrum Jülich and Leiden University.
Snapshot of the ancient stellar flyby. The turquoise particles indicate the TNOs injected into the planet region by the flyby. The perturber star passed through the disk at a perihelion distance of 110 AU on the right-hand side of the picture. Image credit: Pfalzner et al., doi: 10.1038/s41550-024-02349-x.
The solar system planets accumulated from a disk of dust and gas that orbited the young Sun.
Therefore, the planets move close to their common plane on near-circular orbits.
About 3,000 small objects have been observed to orbit the Sun beyond Neptune. Surprisingly, most move on eccentric and inclined orbits.
Therefore, some force must have lifted these trans-Neptunian objects (TNOs) from the disk where they formed and altered their orbits markedly.
“When we think of our Solar System, we usually assume that it ends at the outermost known planet, Neptune,” said lead author Dr. Susanne Pfalzner, an astrophysicist at the Forschungszentrum Jülich.
“However, several thousand celestial bodies are known to move beyond the orbit of Neptune.”
“It is even suspected that there are tens of thousands of objects with a diameter of more than 100 km.”
“Surprisingly, many of these TNOs move on eccentric orbits that are inclined relative to the common orbital plane of the planets in the Solar System.”
In the study, Dr. Pfalzner and her colleagues compared the observed TNO properties with thousands of flyby simulations to determine the specific properties of a possible stellar flyby that reproduces all the different TNO populations, their locations and their relative abundances.
They found that a flyby of a 0.8-solar-mass star at a distance of 110 AU can explain the inclined and eccentric orbits of the known TNOs.
“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,” Dr. Pfalzner said.
“And also objects that move in orbits almost perpendicular to the planetary orbits.”
“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,” said Dr. Amith Govind, also from the Forschungszentrum Jülich.
“This star flew past our Sun at a distance of around 16.5 billion km. 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.”
The astrophysicists also surprisingly found that irregular moons that orbit solar system giant planets on distant, inclined, and eccentric trajectories are, in fact, TNOs that were catapulted into the inner Solar System by the close stellar flyby.
“Some of these objects could have been captured by the giant planets as moons,” said Dr. Simon Portegies Zwart, an astrophysicist at 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. It answers several open questions about our Solar System with just a single cause,” Dr. Pfalzner said.
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Susanne Pfalzner et al. Trajectory of the stellar flyby that shaped the outer Solar System. Nat Astron, published online September 4, 2024; doi: 10.1038/s41550-024-02349-x
Susanne Pfalzner et al. 2024. Irregular Moons Possibly Injected from the Outer Solar System by a Stellar Flyby. ApJL 972, L21; doi: 10.3847/2041-8213/ad63a6
