Rubin's First Dwarf Planet Candidate Hides Beyond Pluto

Rubin's First Dwarf Planet Candidate Hides Beyond Pluto

A bright stranger 46 AU from the Sun

On October 27, the NSF–DOE Vera C. Rubin Observatory announced a large object in the outer solar system, provisionally designated 2026 ED138. It sits about 46 astronomical units from the Sun, beyond Pluto's average distance of roughly 40 AU, making it a trans-Neptunian object. It is Rubin's first dwarf planet candidate, found in data taken earlier in 2026 ahead of the ten-year Legacy Survey of Space and Time.

Between 600 and 1,500 kilometers across

Brightness and assumed surface reflectivity put ED138's diameter at roughly 600 to 1,500 kilometers (370 to 930 miles), potentially larger than Ceres, the smallest recognized dwarf planet. At that size, an icy body should have enough self-gravity to pull itself into a nearly round shape, the key physical test for dwarf planethood. The wide range reflects genuine uncertainty: nobody yet knows how reflective its surface is.

A possible sixth member of an exclusive club

The International Astronomical Union recognizes just five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. ED138 remains a candidate until follow-up measurements pin down its size and shape. If confirmed, it would become the sixth, and one of the largest dwarf planet candidates ever found beyond Neptune.

Found on October 13 by sifting billions of detections

Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian spotted ED138 on the evening of October 13, running Rubin data through search software he developed with colleague Kevin Napier. The code compares billions of moving-object detections to link sightings of the same body. Holman described his first reaction as excited disbelief. Colleague Mario Juric, Rubin's solar system lead scientist at the University of Washington, then found 20 more Rubin observations going back to March 2026, and the team traced the object in archival images all the way to 2016, an arc of more than ten years.

Hidden for 20 years in the Milky Way's glare

ED138 is strangely bright for something discovered so late. The explanation: for the last 20 years it has been drifting across the crowded star fields of the Milky Way as seen from Earth, where moving objects are notoriously hard to pick out and most surveys do not bother looking. Rubin's strategy of imaging the whole sky every few days with the largest digital camera ever built finally caught it. Perched on a 2,682-meter (8,800-foot) mountain in Chile, the telescope began operating in 2025 and will rescan the southern sky for a decade.

A 281-year orbit tilted 25 degrees

ED138's orbit is tilted nearly 25 degrees to the plane of the solar system, classifying it as a hot-classical trans-Neptunian object likely knocked off course by gravitational encounters with Neptune long ago. It takes about 281 years to circle the Sun. Astronomers can already study it with relatively small telescopes and plan spectroscopy to probe its composition; one researcher mused it might even hold nitrogen glaciers like Pluto. Formal numbering and naming await confirmation through the Minor Planet Center process.