The Vera C. Rubin Observatory has identified its first candidate dwarf planet: 2026 ED138, an object in the Kuiper Belt beyond Neptune. It may be among the largest objects yet found in the distant region, but its size is uncertain and it has not been confirmed as a dwarf planet. In a ScienceAlert report, Michelle Starr describes how astronomers found it in new observations and then recognized it in images taken years earlier.
The Kuiper Belt is a broad field of icy objects extending to about 55 astronomical units (AU) from the Sun; Neptune is about 30 AU away. Finding its residents is difficult. They receive little sunlight to reflect and appear faint, while their movement against background stars means a telescope must observe the right part of the sky at the right time. Some distant objects also follow tilted orbits, widening the area astronomers must search. ED138 was additionally set against a crowded stretch of the Milky Way.
Astrophysicist Matthew Holman of the Harvard-Smithsonian Center for Astrophysics identified the object while analyzing Rubin observations on 13 October 2026, using software developed with colleague Kevin Napier, the report says. Rubin had recorded ED138 30 times over 20 nights, from 4 March to 11 July 2026. Searching older records, the team found it in Dark Energy Camera images from February 2016, as well as observations from 2017, 2018, 2019 and 2023. Those earlier detections extended the record used to work out its orbit.
ED138 takes about 281 years to circle the Sun. Its estimated distance ranges from 36.86 AU at its closest to 49.04 AU at its farthest. That places its entire distance range within Pluto’s, which runs from 29.6 to 49.08 AU, although ED138’s orbit is tilted about 24.5 degrees relative to the plane of the Solar System. Its tilt, faintness and crowded background help explain why it could remain unnoticed in earlier searches despite being in Pluto’s general range of distances.
The object’s diameter is estimated at 600 to 1,500 kilometers. That wide range reflects a basic uncertainty: astronomers estimate size from brightness and distance, but a small object with a reflective surface can look as bright as a larger, darker one. Without knowing how reflective ED138 is, its brightness alone cannot settle its size. At the estimated dimensions, it may have enough gravity to pull itself into a roughly spherical shape, a defining feature of dwarf planets. Further observations, particularly in infrared wavelengths, could refine the size estimate and help determine whether it meets the criteria for dwarf planet status.
Rubin’s repeated surveys are designed to find faint objects by returning to large areas of sky and detecting points of light that shift against background stars. Its 3.2-gigapixel camera can image an area equivalent to about 40 full moons in one exposure. The observatory’s decade-long survey, which the report says began in June 2026, is expected to discover thousands of previously unknown outer Solar System objects. A larger sample could help astronomers piece together the region’s history and evolution.
The survey may also help examine broader puzzles, including the hypothetical Planet Nine, proposed as one possible explanation for unusual orbits among some distant objects. ED138’s discovery does not establish that such a planet exists or explain those orbits. For now, it is a promising candidate whose orbit is better mapped than its physical size or status. Repeated observations and infrared measurements will be needed to learn more.