Rubin Observatory Spots 2026 ED138, a Possible Dwarf Planet Beyond Pluto

The Vera C. Rubin Observatory has announced its first candidate dwarf planet: 2026 ED138, a trans-Neptunian object estimated at 600 to 1,500 kilometers in diameter, orbiting roughly 46 astronomical units from the Sun. If follow-up observations confirm a spherical shape, it would rank among the largest dwarf planets known in the outer Solar System. The discovery, made by Matthew Holman of the Harvard & Smithsonian Center for Astrophysics, was announced on 27 October 2026 alongside orbital data from the Minor Planet Center.

• Size range: 600–1,500 km across, based on distance, brightness and assumed surface reflectivity.
• Orbital profile: A 281-year period, ~24.5° inclination and a distance spanning 36.9–49 AU.
• Discovery method: Automated moving-object search across Rubin data, later extended by archival precovery images.

A Large, Tilted World in the Kuiper Belt

2026 ED138 is classified as a “hot” classical trans-Neptunian object—a designation for bodies with notably inclined orbits. Its path is tilted about 24.5 degrees relative to the ecliptic, and it swings from roughly 36.9 AU at perihelion to 49 AU at aphelion. For context, Pluto orbits at an average of about 40 AU.

The size estimate remains the biggest unknown. The 600–1,500 km range derives from the object’s distance, its brightness and an assumption about how much sunlight its surface reflects—a value not yet published. That wide bracket is precisely why infrared observations are needed to pin down its true dimensions and albedo. At the upper end, 2026 ED138 would approach the scale of recognized dwarf planets such as Makemake and Haumea; at the lower end, it would still be a significant find but less historic.

How the Detection Unfolded

The object was flagged on 13 October 2026 by Holman, who was running Rubin observations through search software he developed with Kevin Napier. The code sifts through billions of moving-object detections to identify points that belong to the same body. Holman’s reaction was cautious: “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 had captured 2026 ED138 in 30 detections across 20 nights between March and July 2026. The team then extended the observational arc using archival images from the Dark Energy Camera (DECam) in Chile, reaching back to February 2016. The object had escaped earlier detection largely because it sits near the plane of the Milky Way, where millions of background stars complicate the identification of faint moving points.

“Finding an object this big, roughly 1,000 kilometers across, is something that happens maybe once in a generation. It’s the largest trans-Neptunian object found in about 20 years, and Rubin turned it up while we were still commissioning the telescope.” — Mario Jurić, Rubin Solar System lead scientist

Why This Discovery Matters

Rubin began its ten-year Legacy Survey of Space and Time in late June 2026, mapping the southern sky repeatedly with its 3.2-gigapixel camera. Even before the main survey started, the project had already announced more than 11,000 new asteroids, including hundreds of trans-Neptunian objects. 2026 ED138 is the first candidate dwarf planet from that effort, and it hints at what the full survey may uncover.

Only five dwarf planets are currently recognized by the International Astronomical Union: Pluto, Ceres, Eris, Haumea and Makemake. A confirmed 2026 ED138 would become the sixth. The discovery also underscores how large telescopes and automated search algorithms are reshaping our inventory of the outer Solar System—turning archival data and repeated sky scans into a powerful detection engine.

Until infrared data arrive, the correct label remains “candidate.” But the find already demonstrates that Rubin’s combination of wide-field imaging and sophisticated software can surface objects that hid in plain sight for a decade. If 2026 ED138 proves to be on the larger end of its estimated range, it will stand as one of the most significant outer Solar System discoveries in recent memory.