Discovery of a Dwarf Planet Candidate in the Outer Solar System by the NSF-DOE Rubin Observatory

The orbit of the new candidate dwarf planet beyond Pluto, ED138. (Image Credit: NSF–DOE Vera C. Rubin Observatory/NOIRLab/SLAC/AURA/R)The NSF–DOE Vera C. Rubin Observatory discovered a new dwarf planet candidate, named 2026 ED138, in the outer Solar System beyond the orbit of Pluto. The observational data was announced today (October 27, 2026) here by the International Astronomical Union Minor Planet Center (MPC).If confirmed, ED138 would be among the largest and brightest bodies known in this outer Solar System. A dwarf planet is an object massive enough for its self-gravity to make it nearly spherical. Based on ED138’s distance, brightness and estimated surface reflectivity (albedo), its size is estimated to be in the range between 600 and 1500 kilometers. Icy bodies this large are expected to be round.ED138 was first identified by Matt Holman, my colleague at the Center for Astrophysics | Harvard & Smithsonian. The related Rubin Observatory data was obtained during the run up to the Legacy Survey of Space and Time (LSST). On the evening of October 13, 2026, Holman was running data collected by Rubin earlier in 2026 through a search code he developed with Kevin Napier. The code searches through billions of detections of moving objects and compares them to identify those corresponding to the same object.To verify the validity of the discovery, a detailed search through Rubin’s catalogs and images was subsequently initiated by Mario Jurić, a former postdoc at Harvard’s Institute for Theory & Computation that I directed for 20 years, who is currently a faculty member at the University of Washington and Rubin Solar System Lead Scientist. Jurić found 20 more observations of ED138, dating back from July 11 to March 4, 2026. In total, ED138 was observed 30 times in 20 nights. Additional archival images of ED138 all the way back to 2016, were obtained by the Dark Energy Camera (DECam), mounted on the Victor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observ