Rubin Observatory's Linking Code Found Possible Dwarf Planet 2026 ED138, and Its Author First Checked for a Bug

Nobody spotted 2026 ED138 by squinting at an image. Holman, a Rubin Operations team member at the Center for Astrophysics | Harvard & Smithsonian, runs code that, in Rubin’s words, “is built to look at billions of detections of moving objects, compare them,” and “find the detections that correspond to the same object.” On October 13 it handed him a match he didn’t trust.

“My first reaction was one of excited disbelief,” Holman said in Rubin’s announcement. “Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong.”

Neither had. Mario Jurić, Rubin’s Solar System Lead Scientist at the University of Washington, went digging and found 20 more observations in Rubin’s catalogs. “In all, Rubin’s Simonyi Survey Telescope observed ED138 30 times on 20 nights” between March 4 and July 11. Older pictures from the Dark Energy Camera, turned up by the Asteroid Institute’s ADAM Precovery service, “provided data going all the way back to 2016.”

That backward search is a big part of the trick. Jurić put it this way: “once we identify a new object, we can search back months, and sometimes even decades.” A decade of positions turns a guess into an orbit.

The Minor Planet Center published that orbit in MPEC 2026-U85 on October 27: a period of 281 years, an inclination of 24.47 degrees, and a list of observers that includes Holman, Jurić and Napier on Rubin’s telescope. Rubin says the tilt of “almost 25 degrees” suggests the object “was disturbed from its original orbit through gravitational interactions with Neptune.”

It also explains why ED138 hid so long. It “was found in a region of sky close to the plane of the Milky Way — a crowded region filled with millions of stars.” Picking one faint moving dot out of that is a software problem before it’s a telescope problem.

The size is still a range, and a wide one. Based on distance, brightness and an estimated reflectivity, “the team estimates that its size is between 600 and 1500 kilometers, and icy bodies this large are expected to be round.” Rubin says “observations in the infrared are needed to constrain its exact size and confirm its nature as a dwarf planet.” Until someone points the right instrument at it, Pluto, Ceres, Eris, Haumea and Makemake keep their club to themselves.

If one linking run found a possible dwarf planet within months of the survey starting, more objects like it are likely sitting in Rubin’s catalogs already, waiting on the next pass of the same code. The open question is how many of them hide in the busiest parts of the sky, where the matching is hardest, and whether the infrared follow-up can keep pace with what the software keeps turning up.