On October 13, Matthew Holman found something in Rubin Observatory's data that made him check his own work. A distant object was unusually bright for how far away it appeared to be. It could be a substantial body in the outer Solar System.
His reaction, recalled in the observatory's announcement published by SLAC, was "excited disbelief." He checked whether he or the search code had made a mistake.
The object is now called 2026 ED138. Announced on October 27, it is a dwarf-planet candidate currently about 46 times Earth's distance from the Sun. Its discovery sent astronomers back into photographs taken years before anyone recognized what they contained.
Following it backward
The Rubin Solar System team's account records 30 Rubin observations across 20 nights, from March 4 to July 11, 2026. Older Dark Energy Camera images extended the trail into 2016. Together, the observations span more than ten years.
Finding those earlier appearances is called precovery. The Asteroid Institute's ADAM service, used in the search described by the announcement, works backward from an object's orbit to identify archived exposures that might contain it. Candidate detections still need expert review.
That is a satisfying reversal of the usual discovery story. Once astronomers know where to look, an old photograph can answer a question nobody asked when it was taken. The archive becomes another place to search, with the new orbit guiding the investigation.
ED138 appeared in a crowded patch near the Milky Way's plane. The team says earlier surveys avoided this region or covered it only shallowly. Against that crowded background, the object's movement had to be connected across separate observations.
How large is the thing they found?
The team's technical account gives an estimated diameter of roughly 610 to 1,530 kilometres, depending on how reflective the surface is assumed to be. That is a wide range. Reflected light alone leaves room for a smaller, brighter surface or a larger, darker one.
The team calls for a measurement of reflectivity or an occultation, when the object passes in front of a background star, to narrow its size. The announcement also identifies infrared observations as useful follow-up. Its orbit is much better traced than its physical dimensions.
Why "candidate" still matters
The IAU definition reproduced by NASA requires a dwarf planet to orbit the Sun, have enough gravity to become nearly round, and remain without gravitationally clearing its orbital neighbourhood. It must also be something other than a moon.
Roundness is a physical requirement, so an uncertain size matters to the classification. NASA currently lists five recognized dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. ED138 remains a candidate to join them. The announcement does not establish a newly confirmed ninth planet.
There is also a difference between reading about the discovery and examining all its underlying images. Rubin's release documentation says the submitted Minor Planet Center observations are public, while the wider image and catalogue package is restricted to Rubin data-rights holders. These are preliminary processing products, with calibrations and pipelines that differed during early operations.
Follow-up measurements can now test how large ED138 actually is. The existing photographs have supplied a long trail through the sky; its size and classification still need firmer answers.
Keep investigating with Unexplained
If following a clue back through older records is what draws you into a mystery, explore Unexplained History next. This is a general series recommendation, not an episode about ED138. Choose an investigation from the catalogue and listen from the beginning.
