The solar system's greatest cold case may finally be closing

Space · Published 23 October 2026 · Updated 23 October 2026 Swipe more like this →

In short: For over a century, astronomers have hunted for a massive world hiding in the deep freeze of our outer solar system. Predicted by mathematical models but never seen, this elusive giant could be the key to explaining the strange behavior of distant icy debris.

The search for a hidden planet began in earnest following the discovery of Neptune in 1846, which was inferred from its gravitational influence on Uranus. This methodology led Percival Lowell to propose "Planet X" in 1915 to explain orbital irregularities. However, data from Voyager 2 in the 1980s suggested that those specific orbits could be explained without an extra planet, shifting the focus of modern research much further out. The term Planet X was originally used for a hypothesis developed by Percival Lowell to explain perturbations in Uranus's orbit. The letter x is not intended as the Roman numeral for ten.

In January 2016, Caltech astronomers Konstantin Batygin and Michael Brown proposed a new hypothesis for "Planet Nine." Unlike Lowell's earlier work, this model focuses on orbital peculiarities in small objects in the distant Kuiper Belt. They argue that a planet with a mass between 5 to 10 times that of Earth could account for these patterns. Batygin and Brown did not actually observe any planet when they announced their research. Rather, they contended that a hidden planet might explain peculiar trajectories exhibited by various extreme trans-Neptunian bodies.

This hypothetical world would be roughly Neptune-sized and orbit at an average distance 20 to 30 times farther from the Sun than Neptune. While some researchers recently identified a "good" candidate for this planet in a preprint study, the existence of Planet Nine remains theoretical and contentious within the scientific community. Should it be verified, a full revolution for this sphere would last from 10,000 to 20,000 Earth years. The naming rights remain open; while Batygin and Brown coined "Planet Nine," any formal name must be approved by the International Astronomical Union upon discovery. The hypothetical object would follow a highly elongated path around our sun. If it is eventually found, the person who makes the actual discovery will hold the formal naming rights.

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