Rubin telescope spots an icy world beyond Pluto that could become the sixth dwarf planet
Early estimates put 2026 ED138 at 600 to 1,500 kilometres across, which would make it the largest new body found beyond Neptune in about twenty years.
The Vera C. Rubin Observatory in Chile has discovered an object that may be a new dwarf planet. The International Astronomical Union's Minor Planet Center has given it the provisional designation 2026 ED138, and it orbits the Sun beyond Pluto, according to the US National Science Foundation, which funds the observatory together with the US Department of Energy. VRT NWS and La Libre reported the news on Thursday, based on the observatory's announcement.
The discoverer is astrophysicist Matthew Holman of the Harvard & Smithsonian Center for Astrophysics. According to the University of Washington, on 13 October he ran data Rubin had collected earlier this year through a search program he wrote with a colleague, which compares billions of detections of moving objects. Its brightness and distance immediately showed it was a large object. Holman said his first reaction was disbelief and that he checked whether he or the code had made a mistake. UW astronomer Mario Jurić then found twenty more observations going back to March, and ED138 also shows up in archive images from other telescopes as far back as 2016, allowing its orbit to be reconstructed over more than ten years.
ED138 is currently about 46 astronomical units from the Sun; one unit is the average distance between Earth and the Sun, just under 150 million kilometres. Pluto orbits at an average of 40 units. Based on distance, brightness and likely reflectivity, the team estimates a diameter of 600 to 1,500 kilometres, large enough for its own gravity to have made it round. According to the University of Washington, Jurić described it as the largest object found beyond Neptune in about twenty years. The NSF says it stayed hidden for so long because it sits in a crowded band of the Milky Way, among millions of stars.
ED138 is not a dwarf planet yet. That will take further observations with other telescopes to pin down its exact size and shape. If those succeed, it would join Pluto, Ceres, Eris, Haumea and Makemake, the five dwarf planets the IAU currently recognises. The last recognition dates from 2008, VRT NWS notes. Rubin will image the southern sky again and again for ten years, so more such worlds may turn up, Holman said.