Rubin Observatory spots a candidate for the solar system's sixth dwarf planet

Astronomers working with the Vera C. Rubin Observatory have identified a large, distant object beyond Neptune that the International Astronomical Union's Minor Planet Center has provisionally designated 2026 ED138. Researchers describe it as a dwarf planet candidate, and if the designation holds after further study, it would become the sixth world in that category, joining Pluto, Ceres, Eris, Haumea and Makemake.

The discovery was announced on October 27 in a joint release from the U.S. National Science Foundation and the Department of Energy, which fund the observatory, and detailed further by the University of Washington's news office, home to Rubin's solar system science team. A fuller account appears in NOIRLab's own announcement, the body that operates Rubin's science operations on behalf of NSF and DOE.

What was found

Matthew Holman, an astronomer at the Center for Astrophysics | Harvard & Smithsonian and a member of Rubin's operations team, first noticed the object on the evening of October 13 while reviewing data from the observatory's Simonyi Survey Telescope using a detection code he had built with colleague Kevin Napier. That code sifts through billions of moving-object detections generated by the telescope's nightly scans and links the ones that belong to the same body. Mario Jurić, a University of Washington professor of astronomy who leads Rubin's solar system team, then located 20 additional observations of the object dating back to March. The team subsequently traced the object through archival images from other sky surveys reaching back to 2016, allowing them to reconstruct more than a decade of its orbit in a matter of days — a technique astronomers call precovery, in which a newly found object's calculated path is used to search backward through old survey images for sightings nobody had previously connected to it.

The object had gone undetected for so long partly because of where it sits: close to the plane of the Milky Way, a crowded field of background stars that makes faint, slow-moving bodies difficult to pick out. Rubin's wide field of view and rapid repeat imaging were built in part to solve exactly that problem.

The numbers

• Distance from the sun: roughly 46 astronomical units, compared with Pluto's average distance of about 40 AU.
• Estimated diameter: between 600 and 1,500 kilometers, with Jurić's working estimate near 1,000 kilometers.
• Orbital tilt: about 25 degrees relative to the plane of the solar system, a signature astronomers associate with past gravitational disturbance from Neptune.
• Observational baseline: at least 30 images across 20 nights between March 4 and July 11 this year, plus older archival detections extending the orbit back more than ten years.

Those size estimates depend on assumptions about the object's surface reflectivity, so researchers say infrared observations, which are less sensitive to that uncertainty, will be needed to pin down its true dimensions.

Context: a new survey hitting its stride

The find comes from Rubin's ten-year Legacy Survey of Space and Time, which formally began in June after the telescope's commissioning phase. Even before that formal start, the project had already flagged more than 11,000 new asteroids in April, including roughly 380 previously unknown trans-Neptunian objects. ED138 stands out from that haul because of its size: Jurić called it the largest trans-Neptunian object found in roughly 20 years, a notable claim given that the last comparably sized discoveries date to the mid-2000s identification of Eris, Haumea and Makemake. The object's orbit has already been logged with the International Astronomical Union's Minor Planet Center in its MPCorb catalog, which is maintained with support from NASA's Planetary Defense Coordination Office and the Tamkin Foundation and serves as the clearinghouse astronomers worldwide use to track and confirm newly found solar system bodies.

"Finding an object this big, roughly 1,000 kilometers across, is something that happens maybe once in a generation," Jurić said.

Who is affected, and who is watching

The announcement matters most immediately to the small community of solar system astronomers who track trans-Neptunian objects, for whom a body this large reopens questions about the outer solar system's population and history. It also serves as an early proof of concept for Rubin's broader mission: the observatory is expected to find tens of thousands of new solar system objects over its decade-long survey, and the ability to "precover" an object's past positions in old data, rather than waiting years to build a fresh orbit from scratch, is central to how quickly such discoveries can now be confirmed.

Holman described his own reaction to the detection as one of "excited disbelief," noting that objects of this size are rare enough that his first instinct was to check whether an error in the code, rather than a real object, explained the signal. NSF program director Edward Ajhar and DOE program manager Kathy Turner, who jointly oversee funding for Rubin, both pointed to the discovery as an early demonstration of the telescope's design goals: capturing motion and change across the sky at a scale and speed not previously possible.

What's next

ED138 remains a candidate, not a confirmed dwarf planet. The IAU's criteria require, among other things, enough mass for the object's own gravity to pull it into a round shape, and astronomers currently lack direct measurements of its size and composition. Follow-up observations, particularly in infrared wavelengths that are less dependent on assumptions about surface brightness, are planned at other observatories now that the object's orbit is well enough known to locate it again. Holman has estimated that nailing down its physical size will likely take a few months. In the meantime, the public can track the object's modeled path using an interactive orbit viewer built around the discovery data, and Rubin's team expects the ongoing Legacy Survey of Space and Time to turn up more candidates like it as the ten-year survey continues.