Vera Rubin Observatory's first dwarf planet candidate traced back across a decade of images

The object, provisionally named 2026 ED138, was discovered by Matthew Holman, a Rubin team member and senior astrophysicist at the Center for Astrophysics | Harvard & Smithsonian. Working on October 13 with archived Rubin data gathered earlier in 2026, Holman ran the observations through a search algorithm and spotted the signal. "My first reaction was one of excited disbelief," he said. "Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong."

Rubin's Solar System lead scientist Mario Jurić, of the University of Washington, then combed the observatory's catalogued data and found 20 more detections. In total the Simonyi Survey Telescope caught the object 30 times across 20 nights between March 4 and July 11, and additional images from the Dark Energy Camera extended its observational record back to 2016. That gave astronomers roughly a decade of evidence from which to reconstruct its orbit.

ED138 sits about 46 astronomical units from the Sun, beyond Pluto's average distance of about 40 au, placing it among the trans-Neptunian objects. Its orbit is inclined about 25 degrees from the plane of the Solar System, marking it as a hot-classical TNO, or an object whose path was disturbed by Neptune. Its brightness and distance suggest a diameter between 600 and 1,500 kilometres, which would make it one of the largest candidate dwarf planets found beyond Neptune.

The size estimate carries a caveat: a smaller body with a bright surface reflects as much light as a larger, darker one, and astronomers have not yet pinned down the object's reflectivity. Observations at longer wavelengths could help resolve that ambiguity. If confirmed as a dwarf planet under the International Astronomical Union's definition, ED138 would join the small family of established dwarf planets including Pluto, Eris, Haumea, Makemake and Ceres. “If confirmed as a dwarf planet, ED138 would join a very small family of worlds,” said Holman. “Rubin will image the whole southern sky again and again for ten years, so any others hiding in the places earlier surveys skipped are now within reach.”

The observation was made while Rubin, perched on Cerro Pachón in Chile, was still preparing for its main mission, the decade-long Legacy Survey of Space and Time. The observatory combines an 8.4-metre telescope with a 3,200-megapixel camera, the largest digital camera ever built, and will repeatedly photograph as much of the southern sky as it can every few nights, creating a cosmic movie in which moving or changing objects stand out from static stars.

Edward Ajhar, NSF program director for Rubin Observatory, said Rubin's "super power is the ability to clearly capture movement and change among a vast number of objects and phenomena that humans have never seen before." Kathy Turner, DOE's program manager, said the observatory's strengths make it "an incredible tool for investigating a wide range of astrophysical phenomena, from dark energy to the discovery of this dwarf planet candidate."

Jurić noted the find's wider significance: "Rubin is giving us an unprecedented capacity to explore the sky through a combination of speed, depth, and precision," he said. "Once we identify a new object, we can search back months, and sometimes even decades, finding it in images where nobody knew to look. This ability to both discover and 'precover' objects is transforming how quickly and accurately we can determine their orbits and understand the distant Solar System."

ED138 arrived alongside more than 11,000 new asteroids found in Rubin's pre-survey data. Despite its size, the object is far too small and too close to the Sun to fit the modern Planet Nine hypothesis, which proposes a distant giant to explain the unusual clustering of some extreme outer Solar System orbits.