The Vera C. Rubin Observatory may have found its first dwarf planet. Orbiting beyond Pluto, the object may also be one of the brightest and biggest in that stretch of the solar system.
Matthew Holman, a Rubin operations team member and senior astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, discovered the dwarf planet candidate, now designated 2026 ED138, on the night of Oct. 13. Holman was running Rubin data from earlier this year through a computer program that hunts for moving objects in the solar system. When Holman saw ED138, he immediately noticed how bright and far away it was, and wondered if it was a fluke.
“My first reaction was one of excited disbelief. Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong,” he said in a statement.
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How astronomers confirmed 2026 ED138
The Rubin team investigated whether any other data could confirm what Holman had uncovered. Mario Jurić, Rubin’s solar system lead scientist at the University of Washington, combed through more Rubin data and found 20 more sightings dating back to March 4, 2026. In all, Rubin caught sight of the object 30 times over 20 nights. Researchers also found ED138 in archival images going back to 2016. Those images came from the Dark Energy Camera on the U.S. National Science Foundation’s (NSF) Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile. With this plethora of observations, the team was able to trace the candidate dwarf planet’s orbit over the last 10 years.
The team presented its findings Oct. 27 at the American Astronomical Society’s Division for Planetary Sciences meeting in Spokane, Washington. The International Astronomical Union’s (IAU) Minor Planet Center announced the discovery the same day.
What is a dwarf planet?
If confirmed, ED138 will become the sixth dwarf planet recognized in our solar system. It would join Ceres, Eris, Haumea, Makemake, and most notably Pluto, which was demoted to dwarf planet status in 2006.
Pluto’s demotion traces back to Eris’ 2005 discovery, which revealed an icy world roughly Pluto’s size and forced astronomers to decide whether both or neither were planets. In 2006, the IAU formally defined the term planet for the first time. A planet must orbit the Sun, be massive enough to pull itself into a sphere, and have cleared its orbital path of other debris. A dwarf planet meets the first two criteria but not the third.
It wasn’t the first time a discovery had redrawn the map. Ceres was hailed as a planet when it was found in 1801, and is the largest body in the main belt between Mars and Juptier. As similar objects kept turning up over the following decades, Ceres and its neighbors were reclassified as asteroids — although Ceres is now considered a dwarf planet by the IAU, following the 2006 decision.
ED138 also falls under the classification of trans-Neptunian object (TNO), which are bodies that circle the Sun beyond the orbit of Neptune. Aside from Ceres, the solar system’s other four known dwarf planets are also classified as TNOs
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How big is 2026 ED138?
Based on the data collected so far, researchers know that ED138 orbits roughly 46 astronomical units (AU) from the Sun, or 46 times Earth’s distance from the Sun. That puts it farther out than Pluto, which averages roughly 40 AU. Its brightness and albedo (surface reflectivity) suggest a diameter between 370 and 930 miles (600 and 1,500 kilometers). Further infrared observations will allow scientists to pin down the object’s size more accurately. For now, it appears to be among the largest and brightest dwarf planet candidates found beyond Neptune.
What is the Vera C. Rubin Observatory?
Located on Cerro Pachón in Chile, the Vera C. Rubin Observatory is funded by the NSF and the Department of Energy’s Office of Science. It is operated jointly by NSF NOIRLab and SLAC National Accelerator Laboratory. The observatory officially began its 10-year Legacy Survey of Space and Time (LSST) at the end of June. The survey uses the world’s largest digital camera, the 3,200-megapixel LSST Camera, to sweep the entire southern sky every few nights. It is designed to help scientists tackle four big questions: the nature of dark matter and dark energy, the makeup of the solar system, how the sky changes over time, and the structure of the Milky Way.
“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,” said Holman.
