Rubin Observatory Finds Object Beyond Pluto That May Outsize Ceres

The Vera C. Rubin Observatory has found a large object beyond Pluto that may be bigger than Ceres, but its unusually broad size estimate keeps it in candidate territory. The object, provisionally designated 2026 ED138, could qualify as a dwarf planet once follow-up observations establish how physically large and therefore how likely to be round it really is, according to the observatory’s discovery announcement.

Current estimates place ED138 between 600 and 1,500 kilometers, or 370 to 930 miles, in diameter. Even the lower end would put it near the scale associated with dwarf-planet candidates, while the full range potentially makes it larger than Ceres, the smallest of the five dwarf planets recognized by the International Astronomical Union. The uncertainty is too large for confirmation, however. Smithsonian astrophysicist Matthew Holman said pinning down the size would probably take several months.

A signal hidden among billions of detections

ED138 did not emerge as an obvious new world in a single dramatic image. Holman found it on October 13 while processing Rubin observations collected earlier in 2026 through search software he developed with colleague Kevin Napier. The program compares billions of moving-object detections and attempts to link separate points of light that belong to the same body.

The candidate’s apparent brightness and distance suggested an unusually large object, but those characteristics also warranted checking for a software error or false detection. University of Washington astronomer Mario Jurić subsequently searched Rubin’s catalogs and images, finding 20 additional observations going back to March 4. In total, Rubin had recorded ED138 30 times on 20 nights between March 4 and July 11.

Archival images then changed the quality of the orbital result. Observations from the Dark Energy Camera, recovered through the Asteroid Institute’s ADAM Precovery service, extended the record back to 2016. Astronomers could therefore fit more than a decade of the object’s motion instead of relying only on several months of new Rubin data.

That distinction between finding an object and reconstructing its orbit is central to Rubin’s survey design. Frequent, deep imaging provides the initial detections, while the resulting orbital solution tells researchers where the same faint point should appear in older images. Data that previously looked unremarkable can become useful once astronomers know where to search.

Why earlier surveys missed it

ED138 is currently about 46 astronomical units from the sun, with one astronomical unit equal to the average Earth-sun distance. Pluto’s average distance is about 40 astronomical units. Because ED138 orbits farther from the sun than Neptune, it is classified broadly as a trans-Neptunian object.

Its location on the sky made the search harder. From Earth’s perspective, ED138 spent roughly two decades moving against the Milky Way, where millions of background stars crowd telescope images. Searches for faint moving bodies often avoid such fields because separating a slow-moving point from dense stellar clutter is computationally and observationally difficult.

Rubin is designed to attack that problem through repeated coverage, depth and automated comparison. Located on a 2,682-meter, or 8,800-foot, mountain in Chile, the observatory carries the world’s largest digital camera and began operating in 2025. Its regular monitoring of the Southern Hemisphere sky creates enough time-separated observations for software to isolate motion that a single image cannot reveal reliably.

The volume is already substantial. Before its main Legacy Survey of Space and Time formally began in June 2026, the Rubin team submitted observations of more than 11,000 new asteroids to the International Astronomical Union’s Minor Planet Center, including roughly 380 new trans-Neptunian objects.

Brightness does not settle physical size

The remaining problem is that a distant object’s brightness does not translate directly into diameter. The estimate depends partly on albedo, the fraction of incoming light its surface reflects. A smaller, more reflective body and a larger, darker body can appear similarly bright at the same distance. ED138’s current 600-to-1,500-kilometer estimate combines its distance and brightness with an assumed range of reflectivity.

Infrared observations are needed to narrow that range. Astronomers can also measure distant bodies through stellar occultations when one passes in front of a background star. The duration of the resulting dimming, combined with the object’s known motion, can define a chord across its silhouette; observations from several locations can constrain its projected size and shape. No such measurement has yet been reported for ED138.

Under the International Astronomical Union’s definition, a dwarf planet must orbit the sun, must not be a satellite and must have enough mass for its gravity to pull it into an approximately round shape, while not clearing its orbital neighborhood. Icy bodies within ED138’s estimated size range are expected to be round, but the physical measurement remains the key missing constraint.

If follow-up work confirms dwarf-planet status, ED138 would join Ceres, Pluto, Haumea, Makemake and Eris. It would first receive a permanent number and later a name. For now, Rubin has securely exposed the object and traced its path; measuring the hidden world itself is the next step.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.