Rubin Observatory finds dwarf planet candidate beyond Pluto

Discovery announced

NSF-DOE Vera C. Rubin Observatory has found a dwarf planet candidate beyond Pluto. The object, provisionally designated 2026 ED138, was discovered in Rubin images taken during the run-up to the Legacy Survey of Space and Time. The Minor Planet Center announced the discovery in a Minor Planet Electronic Circular on 4 November.

ED138 is about 46 astronomical units from the Sun, where one astronomical unit is the average distance between Earth and the Sun. Pluto averages 40. Rubin said initial analysis indicates the object may be among the largest and brightest bodies scientists have discovered in that region. The dwarf planet label remains conditional.

Based on the object's distance, brightness and estimated surface reflectivity, or albedo, the team estimates its size at between 600 and 1,500 kilometres. Icy bodies that large are expected to be round. A dwarf planet needs enough mass for its own gravity to pull it into a round shape. Rubin said preliminary analysis places ED138 among the largest dwarf planet candidates.

How the search code found it

Matthew Holman, a Rubin Operations team member and in-kind contributor at the Center for Astrophysics | Harvard & Smithsonian, found the object on the evening of 13 October. He was running Rubin data collected earlier in 2026 through a search code he built with his CfA colleague Kevin Napier. The code looks at billions of detections of moving objects, compares them and finds the ones that belong to the same body. Holman noticed at once that the object's brightness and 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."

Mario Jurić, Rubin's solar system lead scientist at the University of Washington, searched the catalogs and images and found 20 more observations going back to 4 March 2026. In all, the Simonyi Survey Telescope observed 2026 ED138 30 times on 20 nights between 4 March and 11 July. Archival images from the Dark Energy Camera, uncovered by the Asteroid Institute's ADAM Precovery service, extended the record to 2016. The team used those images to trace more than 10 years of the object's orbit.

The orbit is tilted almost 25 degrees off the plane of the Solar System. Rubin said that classifies ED138 as a hot-classical trans-Neptunian object and suggests Neptune's gravity disturbed it from its original orbit.

Why Rubin can do this

Rubin's 8.4-metre telescope carries a 3,200-megapixel camera, an image big enough that displaying one at full resolution would take around 400 Ultra HD televisions. The telescope sweeps the entire southern sky every three nights and collects about 10 terabytes of data a night. The observatory began its 10-year Legacy Survey of Space and Time on 30 June 2026.

Rubin will repeat the southern-sky sweep for a decade, building a time-lapse of the changing sky. Holman and Jurić presented the results at a press conference at the 58th Annual Division for Planetary Sciences meeting in Spokane, Washington. Rubin Observatory is jointly funded by the National Science Foundation and the Department of Energy's Office of Science.