In the remote reaches of our Solar System, clusters of rocks orbit in unusual patterns that have long puzzled astronomers. These orbits, which some scientists have attributed to a hypothetical Planet Nine, could potentially be explained by a different cosmic event altogether.
New simulations led by astrophysicists from Forschungszentrum Jülich in Germany suggest that an ancient stellar flyby might be the culprit behind these strange orbits.
Despite extensive searches, Planet Nine, the theorized massive planet on the fringes of our Solar System, has remained elusive. An alternative explanation offered by this new research proposes that billions of years ago, a massive object, possibly a star, passed close enough to the Solar System to gravitationally disturb the orbits of objects in the outer regions.
Astrophysicist Susanne Pfalzner and her team carried out over 3,000 computer simulations to explore this possibility. These simulations aimed to observe the effects that stars of various masses and distances could have on the outer Solar System’s objects as they passed by.
Dr. Amith Govind from Forschungszentrum Jülich explains, “The best match for today’s outer Solar System that we found with our simulations is a star slightly lighter than our Sun, about 0.8 solar masses. This star flew past our Sun at a distance of around 16.5 billion kilometers.” This distance is approximately 110 times that from Earth to the Sun, and nearly four times the distance of Neptune from the Sun.
Typically, most material within the Solar System orbits in a more-or-less flat disk configuration. This disk formation resulted from the material around a young Sun flattening out over time as it spun, akin to pizza dough expanding when spun. This material eventually formed the planets, asteroids, and moons of our Solar System.
However, the outer Solar System is markedly different. Objects known as trans-Neptunian objects (TNOs) orbit the Sun at unusually inclined angles, with some even orbiting almost perpendicularly to the Sun’s equator. Such orbits have been hypothesized to be influenced by a planet up to five times Earth’s mass.
Given that the proximity of other stars changes over time, Pfalzner and her colleagues conducted simulations to mirror how varying stars might affect these outer objects. They concluded that a star smaller than the Sun, passing by the system, could have produced the current misaligned orbits. This stellar flyby might also explain the mysterious orbits of objects like 2008 KV42 and 2011 KT19, which revolve in the opposite direction to the planets at almost perpendicular angles.
Further, the simulations suggest that up to 7.2 percent of the original TNO population could have been flung inward towards the Sun. “Some of these objects could have been captured by the giant planets as moons,” notes Simon Portegies Zwart from Leiden University in the Netherlands, indicating that this could justify why outer planets have two distinct types of moons.
While this study is not conclusive, it offers a plausible and simple explanation for the eccentric orbits observed today. Other reasons for not finding Planet Nine include its faintness and distance. Additionally, current technology limitations might result in selection bias in our data.
“The beauty of this model lies in its simplicity,” Pfalzner remarks. “It answers several open questions about our Solar System with just a single cause.” The research findings have been published in the journals Nature Astronomy and The Astrophysical Journal Letters.