Astronomers have long puzzled over the strange, tilted orbits of celestial objects in the outer Solar System, hypothesizing that a hidden, massive “Planet Nine” could be influencing these peculiar trajectories. However, a new study suggests an alternative explanation: an ancient passing star.
Researchers from Forschungszentrum Jülich in Germany, led by astrophysicist Susanne Pfalzner, propose that billions of years ago, a star slightly smaller than the Sun passed near our Solar System. This stellar encounter, they argue, could have gravitationally disturbed the orbits of trans-Neptunian objects (TNOs), clusters of icy rocks beyond Neptune, causing the odd orbits we see today. Some of these rocks could have been flung inward, potentially being captured by the giant planets as moons.
The team’s simulations suggest that the star, about 0.8 times the mass of our Sun, passed at a distance of 16.5 billion kilometers (roughly 110 times the Earth-Sun distance). This stellar flyby may have tilted the orbits of TNOs, resulting in objects orbiting at highly inclined angles including some that orbit almost perpendicularly to the Sun’s equator.
“We believe that this close encounter with a passing star could explain the gravitational quirks we observe in the outer reaches of the Solar System,” said Amith Govind, co-author of the study. “It provides an elegant explanation for these haphazard orbits without the need for an unseen planet.”
Search for Planet Nine
For years, the strange orbits of distant objects like 2008 KV42 and 2011 KT19 which orbit the Sun in the opposite direction of planets have been linked to the possible presence of Planet Nine. The elusive planet was theorized to be up to five times the size of Earth, lingering in the dark depths of the Solar System, yet it has never been found despite extensive searches.
Pfalzner’s team ran over 3,000 simulations, adjusting the size and distance of passing stars to see how they might impact the Solar System’s outer regions. They found that a stellar flyby could explain not only the skewed orbits of TNOs but also why some moons around the gas giants have such varied origins.
“This passing star could explain the two distinct types of moons we see around the outer planets,” explained Simon Portegies Zwart from Leiden University, another study author. “Some of these moons might have been flung inward by the star’s gravity and later captured by the planets.”
Planet Nine: Still a Mystery?
While the study offers a compelling alternative, it doesn’t fully rule out the existence of Planet Nine. The planet could simply be very far away, extremely dim, or evading detection due to current technological limitations. Additionally, data from the outer Solar System may be incomplete, leaving room for future discoveries.
Nevertheless, the stellar flyby hypothesis is gaining traction as a simple, plausible explanation for the unusual orbits. “The beauty of this model lies in its simplicity,” Pfalzner remarked. “It answers several open questions about our Solar System with just a single cause.”
A New Look at the Early Solar System
The notion that stars once passed close to the Sun is not implausible. Stars typically form in crowded nurseries, where they drift apart over time. This study hints at the early dynamics of our Solar System, suggesting that it may have been shaped not just by its own formation but by interactions with nearby stars.
As the debate over Planet Nine continues, this new research opens up exciting possibilities for understanding the forces that shaped our Solar System billions of years ago. Could a distant star, rather than a hidden planet, be the key to unlocking one of astronomy’s great mysteries? Only time and further research will tell.
The study has been published in Nature Astronomy and The Astrophysical Journal Letters.
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