A new candidate beyond Pluto
Astronomers announced on 27 October the discovery of 2026 ED138, a trans-Neptunian object classified as a dwarf planet candidate. Rubin Observatory observations place it about 46 astronomical units from the Sun — well beyond Pluto's average of 40 au. From its brightness, distance and estimated surface reflectivity, the team puts its diameter between 600 and 1,500 kilometres. Icy bodies that large are expected to be rounded by their own gravity, which is precisely what makes an object a dwarf planet.
If confirmed, ED138 would be among the largest objects ever found in this region and would join the five dwarf planets the International Astronomical Union recognises: Pluto, Ceres, Eris, Haumea and Makemake.
Found in a field nobody was searching
Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian, who works on Rubin's algorithms for distant moving objects, made the find. On the evening of 13 October he ran 2026 data through a search code he built with colleague Kevin Napier, which compares billions of detections and picks out those belonging to the same object.
"My first reaction was one of excited disbelief," Holman said. "Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong." The check held: Mario Jurić of the University of Washington found 20 further observations in Rubin's catalogues and images, going back to 4 March 2026. All told, Rubin imaged ED138 thirty times across twenty nights between 4 March and 11 July.
Archival images from the Dark Energy Camera, surfaced by the Asteroid Institute's ADAM precovery service, extend the record to 2016 — enough to trace more than ten years of orbit, which is what separates a real object from an artefact.
Why it stayed hidden
ED138 sits in a region of sky close to the plane of the Milky Way, where millions of stars crowd the background. To most telescopes it was indistinguishable from that noise. Rubin's repeated, continuous imaging of the entire southern sky, combined with algorithms that sift millions of points of light, is what brought it out.
Its orbit is tilted almost 25 degrees off the plane of the solar system, making it a "hot-classical" trans-Neptunian object — one disturbed from its original path by gravitational interaction with Neptune. Objects like this are witnesses to the early migration of the giant planets.
The start, not the finish
Rubin began collecting early data ahead of the Legacy Survey of Space and Time, which formally started in June 2026. In April, before the survey began, the Rubin solar system team submitted more than 11,000 new asteroids to the Minor Planet Center, including roughly 380 new TNOs — a batch it then called "the tip of the iceberg".
"Rubin Observatory's superpower is the ability to clearly capture movement and change among a vast number of objects and phenomena that humans have never seen before," said Edward Ajhar, NSF program director for Rubin.
Dwarf planet status is not yet confirmed. Infrared observations are needed to pin down size and reflectivity. The images are available to data rights holders on the Rubin Science Platform, the public can follow the orbit in Orbitviewer, and the results were presented at the 58th meeting of the Division for Planetary Sciences in Spokane.
What actually matters
The headline is not the tally of dwarf planets but the proof that Rubin is delivering what it promised: a decade-long, repeated record of the southern sky in which even the crowded regions earlier surveys skipped become searchable. As Jurić put it, the ability to both discover objects and precover them in old images is changing how quickly and how confidently orbits can be determined.
