A University of Washington astronomy professor helped confirm what could become the sixth recognized dwarf planet in our solar system.
Mario Jurić, a UW professor of astronomy and lead scientist for the Rubin Observatory's solar system team, presented the findings Oct. 27 at the 58th annual Division for Planetary Sciences meeting in Spokane alongside Matthew Holman, a senior astrophysicist at the Center for Astrophysics | Harvard & Smithsonian. Holman first spotted the object, provisionally named 2026 ED138, on the evening of Oct. 13 while running earlier Rubin data through a search algorithm he developed with colleague Kevin Napier, according to a NOIRLab press release.
The data Holman analyzed was collected during the observatory's commissioning phase, before its 10-year Legacy Survey of Space and Time (LSST) formally began in June 2026. The telescope observed ED138 30 times on 20 nights between March 4 and July 11, according to NOIRLab. Archival images from other surveys extended the object's orbital record back to 2016.
The icy body orbits the sun beyond Pluto at roughly 46 astronomical units, or 46 times the average Earth-to-sun distance. The team estimates its diameter at between 600 and 1,500 kilometers. Icy bodies that large are expected to be round, a key requirement for dwarf planet classification by the International Astronomical Union (IAU).
"Finding an object this big, roughly 1,000 kilometers across, is something that happens maybe once in a generation," Jurić said, according to UW News. "It's the largest trans-Neptunian object found in about 20 years, and Rubin turned it up while we were still commissioning the telescope."
ED138 had gone undetected because it sits in a crowded region of sky near the plane of the Milky Way, where millions of background stars obscure faint moving objects. The observatory's algorithms sift through billions of light sources to pick out small moving bodies.
If confirmed, ED138 would join Pluto, Ceres, Eris, Haumea and Makemake on the IAU's official dwarf planet list. That list has not changed since 2008, GeekWire reported. Infrared observations are still needed to pin down ED138's exact size and confirm it is round.
The discovery builds on a productive stretch for the UW-based Rubin solar system team. In April, before LSST formally began, the team announced observations of more than 11,000 new asteroids processed with software developed at UW's DiRAC Institute. That haul included roughly 380 trans-Neptunian objects (TNOs), adding to about 5,000 TNOs discovered over the previous three decades.
Scientists worldwide can now organize follow-up observations. The public can explore ED138's orbit on Orbitviewer, an interactive web app.
