Mysterious distant object found at edge of solar system
A team of astronomers has identified a peculiar, icy celestial body lurking at the outermost fringes of our solar system, potentially significant enough to be classified as a dwarf planet. The discovery, detailed in a study published this week, introduces 2017 OF201, an object approximately 435 miles (700 kilometers) in diameter, orbiting the Sun in a highly eccentric path that extends up to 1,600 astronomical units (AU)—an AU being the average distance from Earth to the Sun, about 93 million miles (150 million kilometers). This places 2017 OF201 far beyond the Kuiper Belt, a distant region beyond Neptune teeming with icy objects, comets, and other small bodies.
The discovery was officially verified by the Minor Planet Centre of the International Astronomical Union (IAU), which catalogs and tracks such findings. The confirmation stems from meticulous observations gathered over seven years using advanced telescopes at observatories in Chile, including the Cerro Tololo Inter-American Observatory, and Hawaii, such as the Subaru Telescope on Mauna Kea. These observatories employed powerful optical and infrared instruments to detect and track the faint light reflected by 2017 OF201, given its immense distance and dim appearance.
Leading the study, astrophysicist Sihao Cheng from the Institute for Advanced Study in Princeton, New Jersey, emphasized the object’s potential to reshape our understanding of the solar system’s outer boundaries. “This object resides in a region long thought to be a near-empty void,” Cheng explained. “Its discovery suggests that the distant solar system may be far more populated and dynamic than previously assumed, offering clues to its formation and evolution.”
Currently positioned at 90.5 AU from the Sun—over 90 times farther than Earth and nearly twice Pluto’s average distance of 39 AU—2017 OF201 follows an extraordinarily elongated orbit. This orbit, taking roughly 25,000 years to complete a single revolution around the Sun, ranks it among the most distant observable objects in our solar system. For perspective, Neptune orbits at about 30 AU, and even the Kuiper Belt rarely extends beyond 50 AU. The object’s extreme path suggests it may have been gravitationally perturbed in the distant past, possibly by a massive planet or other significant celestial body, causing it to migrate to its current trajectory.
While its exact composition remains uncertain due to the challenges of analyzing such a remote object, Cheng speculates that 2017 OF201 is likely composed of ices such as water, methane, and ammonia, mixed with rocky material—similar to other trans-Neptunian objects (TNOs) in the Kuiper Belt. “Its icy nature is inferred from its location and the characteristics of similar bodies,” Cheng noted. However, advanced spectroscopy, which analyzes the light spectrum to determine composition, has been difficult to perform due to the object’s faintness and distance.
The object’s size, estimated at 435 miles in diameter, places it in the realm of dwarf planet candidates. By comparison, Ceres, the smallest recognized dwarf planet, measures about 590 miles (950 kilometers) across, while Pluto spans roughly 1,470 miles (2,370 kilometers). Although 2017 OF201 is smaller than Ceres, its size suggests it could meet the IAU’s criteria for a dwarf planet: a celestial body that orbits the Sun, is spherical or nearly so due to its own gravity, but has not cleared its orbit of other debris. Unlike planets, which dominate their orbital paths, dwarf planets coexist with other objects in their vicinity. The five officially recognized dwarf planets are Ceres, Pluto, Haumea, Makemake, and Eris. Determining 2017 OF201’s shape—whether it has achieved hydrostatic equilibrium (a near-spherical form)—remains challenging due to its distance, which limits high-resolution imaging.
With an estimated mass 20,000 times smaller than Earth’s and approximately 50 times less than Pluto’s, 2017 OF201 is a lightweight in planetary terms. Its discovery, however, carries heavyweight implications for our understanding of the solar system’s structure. Most TNOs exhibit clustering in their orbits, a pattern that has fueled speculation about the existence of a hypothetical “Planet Nine” or “Planet X,” a massive, unseen planet thought to influence the orbits of distant objects. Intriguingly, 2017 OF201’s orbit does not align with this clustering, making it an outlier. “This object’s unique trajectory challenges the Planet Nine hypothesis,” Cheng said. “It suggests that the dynamics of the outer solar system may be more complex than a single massive planet can explain.”
The discovery of 2017 OF201 underscores the limitations of current observational technology. Detecting objects beyond 150 AU is extraordinarily difficult, as their faintness requires long exposure times and large telescopes. Cheng estimates that hundreds, if not thousands, of similar-sized objects could remain undetected in the solar system’s outer reaches. “This single discovery is likely the tip of the iceberg,” he said. “The region beyond Neptune is not a barren wasteland but a frontier filled with undiscovered worlds.”
The study also highlights the importance of ongoing and future missions to explore the outer solar system. Projects like the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), set to begin operations in 2025, could reveal more objects like 2017 OF201, potentially transforming our understanding of the solar system’s boundaries. For now, 2017 OF201 stands as a tantalizing clue to the mysteries of the cosmic frontier, prompting astronomers to rethink the distribution and history of objects in the distant solar system.
“This discovery is a reminder of how much we still have to learn about our own cosmic backyard,” Cheng concluded. “Each new object we find is a piece of the puzzle, bringing us closer to understanding the solar system’s past and present.”