The object, provisionally designated 2026 ED138, was announced by the International Astronomical Union’s Minor Planet Center on 27 October. Researchers estimate its diameter at between 600 and 1,500 kilometres, although its precise size and classification remain unconfirmed. The estimate reflects uncertainty about surface reflectivity.
Located approximately 46 astronomical units from the Sun, ED138 is about 46 times farther from our star than Earth. Its apparent brightness, despite that distance, suggests a icy body that may be massive enough for gravity to shape it into a sphere.
Confirmation would place it alongside the five dwarf planets formally recognised by the International Astronomical Union: Pluto, Ceres, Eris, Haumea and Makemake. Astronomers emphasise that additional measurements, particularly at infrared wavelengths, are needed before determining whether ED138 qualifies. A dwarf planet must be nearly round but must not have cleared its orbital neighbourhood of other objects.
Matthew Holman, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and a member of Rubin’s operations team, identified the object on 13 October while analysing observations gathered earlier this year. He used software developed with colleague Kevin Napier to match detections of moving objects across astronomical datasets.
Holman initially questioned the result because objects of comparable brightness and distance are uncommon. Subsequent checks found additional detections, allowing the research team to establish that the signal represented a real solar system body rather than an error in the analysis.
Mario Jurić, a University of Washington astronomy professor and Rubin’s solar system lead scientist, located further observations in the telescope’s records. Altogether, Rubin had photographed ED138 30 times on 20 nights between 4 March and 11 July.
Researchers also identified the object in older images taken with the Dark Energy Camera on the Víctor M. Blanco telescope in Chile. Those archival detections extend back to 2016, providing more than a decade of measurements for calculating its orbit.
The extended observational record is important because distant objects move slowly against the background stars. A longer sequence of positions helps astronomers distinguish their trajectories and reduces uncertainty about where they will appear during subsequent observations.
ED138 follows an orbit inclined by approximately 24 degrees relative to the main plane of the solar system. Researchers classify it as a hot-classical trans-Neptunian object, a population whose orbital characteristics indicate that gravitational interactions helped shape its present distribution.
Its trajectory may preserve evidence of Neptune’s influence during the solar system’s evolution. Scientists are examining whether its orbit lies in a gravitational resonance with Neptune, although that relationship has not yet been established conclusively. The analysis places its average orbital distance at approximately 43 astronomical units.
The discovery was unusually difficult because ED138 appeared close to the Milky Way’s crowded galactic plane. Dense background star fields can obscure the faint movements of distant bodies, creating challenges for conventional searches.
Rubin’s wide-field imaging and automated detection systems enabled scientists to distinguish the object’s movement from millions of background sources. Its camera and repeated observations are designed to identify changes across large areas of the southern sky.
The discovery also provides an early demonstration of the observatory’s Legacy Survey of Space and Time, which began in June. The programme is designed to survey the southern sky repeatedly over ten years, generating records of asteroids, distant solar system bodies and changing astronomical phenomena.
Before the survey formally began, Rubin researchers submitted observations of more than 11,000 newly identified asteroids to the Minor Planet Center in April, including approximately 380 trans-Neptunian objects.
A scientific paper released on 28 October places ED138 among the ten intrinsically brightest known trans-Neptunian objects. Its authors estimate that several comparably bright bodies could remain undiscovered, particularly in regions complicated by dense stellar backgrounds.
Further observations may use ground-based spectroscopy, infrared measurements and space telescopes to investigate the object’s surface, reflectivity and dimensions. Its current brightness makes it accessible to several instruments capable of refining those measurements. Measurements of its colour and rotation could characterise its surface.
The observatory is jointly funded by the US National Science Foundation and the Department of Energy’s Office of Science. Its Simonyi Survey Telescope operates from Cerro Pachón in northern Chile.
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