Rubin Observatory identifies its first dwarf-planet candidate
Astronomers linked repeated sightings of 2026 ED138 with images dating to 2016, allowing them to reconstruct more than ten years of its orbit.
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Astronomers linked repeated sightings of 2026 ED138 with images dating to 2016, allowing them to reconstruct more than ten years of its orbit.
Vera C. Rubin Observatory announced its first dwarf-planet candidate, provisionally designated 2026 ED138, in an October 27 announcement. The team estimates the distant object is 600–1,500 kilometres across, but its dwarf-planet classification remains unconfirmed.
The discovery grew out of observations collected around the launch of Rubin’s decade-long sky survey, which began in June 2026. ED138 appeared against a crowded field of stars near the plane of the Milky Way. Rubin said this crowded background likely explains why earlier searches missed it.
On October 13, 2026, Matthew Holman, a senior astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and a Rubin Operations team member, identified ED138 in earlier Rubin data. He used search code that compares billions of moving-object detections and links those belonging to the same object. Its brightness and distance suggested an unusually large body.
Mario Jurić, Rubin’s Solar System Lead Scientist and a University of Washington faculty member, found 20 additional observations in Rubin’s catalogues and images. In total, Rubin’s Simonyi Survey Telescope recorded ED138 30 times on 20 nights between March 4 and July 11, 2026.
The Asteroid Institute’s ADAM Precovery service then recovered Dark Energy Camera images containing ED138 that dated back to 2016. Combining those older positions with Rubin’s detections let the team trace more than ten years of the object’s path and reconstruct a preliminary orbit. The Minor Planet Center announced the discovery in an electronic circular on October 27, Rubin said.
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Its orbit tilts almost 25 degrees from the Solar System’s plane. Rubin classifies it as a “hot-classical” trans-Neptunian object, an orbital category rather than a description of temperature. The team says the tilt suggests that gravitational interactions with Neptune disturbed its original orbit.
Analysis
The dwarf-planet definition adopted in 2006 covers bodies that directly orbit the Sun, are rounded by their own gravity and have not cleared their orbital neighbourhoods of smaller bodies. Being beyond Neptune alone does not make an object a dwarf planet.
ED138’s estimated size comes from its brightness, distance and assumed surface reflectivity, known as albedo—not a directly measured diameter. The team expects icy bodies in the estimated size range to be round, which makes ED138 a candidate. Rubin says studying such bodies’ sizes, compositions and orbits helps researchers investigate how the giant planets moved and scattered smaller objects early in the Solar System’s history.
Rubin says infrared observations are needed to narrow ED138’s size estimate and confirm its dwarf-planet nature. It has not given a date for those observations. Identifying the object now allows researchers to organize that follow-up.
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