Rubin Observatory Spots a Giant Icy World Hiding Beyond Pluto

Image: Collected

The Vera C. Rubin Observatory has discovered a massive dwarf planet candidate lurking in the crowded star fields beyond Neptune — an icy world that could rank among the largest ever found in the outer solar system.

The object, provisionally named 2026 ED138, was announced by the International Astronomical Union’s Minor Planet Center on Oct. 27. It lies about 46 astronomical units from the Sun — farther out than Pluto — and is estimated to be between 600 and 1,500 kilometers wide. At that size, scientists expect it to be round, the key requirement for dwarf-planet status.

Found in a crowded sky

Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian spotted the object on Oct. 13 while scanning Rubin data with search software he built with colleague Kevin Napier. The tool sifts through billions of detections to connect faint moving points into a single orbiting body.

“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.”

Rubin imaged 2026 ED138 30 times across 20 nights between March 4 and July 11. Archival images from the Dark Energy Camera in Chile, recovered through the Asteroid Institute’s ADAM Precovery service, extend observations back to 2016 — giving researchers more than a decade of orbital data.

The object had likely gone unnoticed because it sits near the Milky Way’s plane, where millions of background stars can mask a faint, slow-moving world. Its orbit is tilted nearly 25 degrees from the solar system’s main plane, suggesting Neptune’s gravity flung it onto its unusual path long ago.

Size still a mystery

The wide size estimate reflects uncertainty about the object’s surface brightness. “It could either be a very reflective object that’s smaller or a very dark object that’s bigger,” Holman told GBH.

Infrared observations, stellar occultations, thermal measurements and light curves could help determine its true size and shape. NASA’s James Webb Space Telescope is among the instruments capable of providing the needed infrared data.

More discoveries ahead

If confirmed, 2026 ED138 would join Pluto, Ceres, Eris, Haumea and Makemake as recognized dwarf planets. And it may be only the beginning: in April, Rubin’s team submitted more than 11,000 new asteroid discoveries, including roughly 380 trans-Neptunian objects, even before its main decade-long sky survey began in June.

“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,” Holman said.