A giant icy world hiding beyond Pluto could be our sixth dwarf planet

An enormous icy world discovered beyond Pluto could soon become the sixth officially recognized dwarf planet in our solar system. The object, temporarily named 2026 ED138, was identified by an astronomer analyzing observations from the Simonyi Survey Telescope at the NSF-DOE Vera C. Rubin Observatory. Early estimates suggest it ranks among the largest dwarf planet candidates ever found beyond Neptune. If its status is confirmed, it would join the five dwarf planets currently recognized by the International Astronomical Union: Pluto, Ceres, Eris, Haumea and Makemake. The discovery was announced at a press conference featuring Matthew Holman, a senior astrophysicist at the Harvard & Smithsonian Center for Astrophysics, and Mario Jurić, a University of Washington astronomy professor who leads Rubin's solar system team. Holman made the initial discovery, while Jurić helped gather the additional evidence needed to verify it. "If confirmed as a dwarf planet, ED138 would join a very small family of worlds," Holman said. "Rubin will image the whole southern sky again and again for 10 years, so any others hiding in the places earlier surveys skipped are now within reach." "Finding an object this big, roughly 1,000 kilometers across, is something that happens maybe once in a generation," Jurić said. "It's the largest trans-Neptunian object found in about 20 years, and Rubin turned it up while we were still commissioning the telescope." How Astronomers Discovered the Hidden World The breakthrough occurred on the evening of October 13, when Holman examined observations gathered by Rubin earlier in 2026. He was using specialized software developed with his colleague Kevin Napier to search for previously unidentified objects moving through the solar system. The program compares billions of telescope detections, looking for individual points of light that appear in multiple observations and move in ways that suggest they belong to the same object. When the software identified ED138, Holman quickly realized that its apparent brightness and enormous distance pointed to something unusually large. "My first reaction was one of excited disbelief," Holman said. "Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong." To check the finding, Jurić examined Rubin's stored observations and located 20 additional detections of ED138, including some captured as early as March 2026. The researchers then searched historical records from other astronomical surveys and identified images of the object dating back to 2016. Together, these observations allowed the team to reconstruct more than a decade of the distant world's orbital motion, strengthening the evidence that the discovery was genuine. Could This Be Our Solar System's Sixth Dwarf Planet? ED138 currently lies approximately 46 astronomical units (au) from the Sun. One astronomical unit represents the average distance separating Earth and the Sun. By comparison, Pluto's average orbital distance is about 40 au. Because ED138 travels beyond Neptune, astronomers classify it as a trans-Neptunian object (TNO). These distant bodies are valuable to scientists because their physical properties and orbital paths preserve clues about the solar system's distant past. By examining their sizes, compositions, and movements, researchers can better understand how the solar system developed, how its giant planets shifted positions, and how smaller bodies were redistributed during that process. However, being a trans-Neptunian object does not automatically make ED138 a dwarf planet. An essential requirement is that the object's gravity must be strong enough to shape it into a nearly round body. Using measurements of ED138's brightness and distance, along with estimates of how much sunlight its surface reflects, astronomers calculate that it could measure anywhere from 600 to 1,500 kilometers across. An icy object within that size range would generally be expected to have enough gravity to become round. Nevertheless, scientists will need more observations to establish ED138's actual dimensions and determine whether it qualifies as a dwarf planet. Why Such a Massive World Remained Undiscovered One of the most surprising aspects of the discovery is how an object this large managed to escape detection for so long. ED138 occupies a region of the sky near the plane of the Milky Way, where millions of stars create an exceptionally crowded background. Against that dense field of light, the distant object's faint movement was difficult for earlier telescopes and surveys to distinguish. Rubin Observatory has an important advantage. Its ability to repeatedly observe the Southern Hemisphere sky, combined with advanced computer algorithms, makes it possible to search enormous quantities of astronomical data for subtle changes in the positions of objects. By examining millions or even billions of individual light detections, these systems can recognize the movement of small solar system bodies that might otherwise disappear into the surrounding stars. This capability was essential to finding ED138 in such a challenging part of the sky. Rubin Observatory Could Reveal More Hidden Worlds The newly identified object adds to a growing collection of solar system discoveries made by Rubin Observatory as it began gathering observations ahead of its ambitious Legacy Survey of Space and Time (LSST), which officially started in June 2026. Even before the survey began, the observatory was producing remarkable results. In April, Rubin's solar system researchers reported observations of more than 11,000 previously unknown asteroids, including approximately 380 new trans-Neptunian objects. The team described that enormous collection of discoveries as "the tip of the iceberg," anticipating that the full survey would uncover many more objects. The identification of ED138 offers an early indication of just how significant those future findings could be. Rubin's combination of extensive sky coverage, sensitivity, and rapid observations is also changing how astronomers investigate new discoveries. Once an object has been identified, researchers can search older telescope images to find instances when it was photographed but went unrecognized. "Rubin is giving us an unprecedented capacity to explore the sky through a combination of speed, depth, and precision," Jurić said. "And the discovery is just the beginning: Once we identify a new object, we can search back months, and sometimes even decades, finding it in images where nobody knew to look. This ability to both discover and 'precover' objects is transforming how quickly and accurately we can determine their orbits and understand the distant solar system." As Rubin continues surveying the southern sky over the next decade, astronomers expect its observations to uncover additional objects that previous searches missed, potentially revealing more large, icy worlds in the outer solar system. Anyone interested in exploring ED138 can view its location and orbit using Orbitviewer, an interactive web-based application that displays the solar system in three dimensions and allows users to explore its structure in real time. Operations of the Vera C. Rubin Observatory are funded by the U.S. National Science Foundation and the U.S. Department of Energy's Office of Science.