Rubin Observatory discovers icy dwarf planet candidate 2026 ED138 beyond Neptune

The object, 2026 ED138, could be one of the largest bodies known in the outer solar system, but its size and status still need confirming.

The NSF–DOE Vera C. Rubin Observatory has found its first candidate dwarf planet: an icy body orbiting the Sun beyond Neptune, currently about 46 times farther from the Sun than Earth is. The object, provisionally designated 2026 ED138, was announced on October 27 by the observatory and the International Astronomical Union’s Minor Planet Center.

Its exact dimensions remain undetermined. Should the highest projections prove accurate, this object will count as one of the most massive entities ever found past Neptune, with researchers noting that it likely possesses sufficient mass to achieve a spherical form through its own gravity, which serves as the primary physical criterion for dwarf planet classification.

How the object was found

Matthew Holman, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, spotted the object on October 13 while running Rubin data gathered earlier in the year through software he built with colleague Kevin Napier to link detections of moving objects. “My first reaction was one of excited disbelief,” Holman said in the observatory’s announcement.

Mario Jurić of the University of Washington, Rubin’s Solar System lead scientist, then searched the observatory’s catalogs and images for more sightings. In all, Rubin’s Simonyi Survey Telescope in Chile recorded the object 30 times on 20 nights between March 4 and July 11, 2026. Archival images from the Dark Energy Camera on the Víctor M. Blanco telescope, also in Chile, extend the record back to 2016, giving a track of more than 10 years. The Minor Planet Center circular lists both sets of observations and a preliminary orbit.

The object was not hiding because it is faint. It sits far in the southern sky and close to the plane of the Milky Way, where dense star fields make moving points hard to pick out. Holman told CNN that this location rules out many northern telescopes and that astronomers rarely search the Milky Way for such objects.

What is understood regarding its scale and orbit

Astronomers cannot yet measure the object’s diameter directly. They infer it from its brightness and distance, an estimate that hinges on how much sunlight the surface reflects: a dark surface implies a larger body, a bright one a smaller body. Hence the wide announced range of 600 to 1,500 kilometers.

In a preprint posted on arXiv, Holman, Napier, Jurić and their colleagues favor reflectivities that would put the object at roughly 610 to 970 kilometers wide. Even the low end exceeds the size of about 450 kilometers above which the team expects an icy body to be round. The paper notes that only nine known trans-Neptunian objects, including Pluto, Eris, Makemake and Haumea, are intrinsically brighter. The team and the Minor Planet Center report slightly different brightness values, a gap that further observations should resolve.

The orbit is better constrained. The object circles the Sun about every 281 years at an average distance of roughly 43 astronomical units (one AU is the Earth–Sun distance). It ranges from about 37 AU at its closest to about 49 AU at its farthest, on a path tilted around 24 degrees to the plane of the planets. That tilt places it among the so-called hot classical Kuiper Belt objects. The team suggests Neptune’s gravity pushed it onto this inclined orbit long ago.

The orbit lies near a 12:7 resonance with Neptune, the same one occupied by Haumea, but simulations have not settled whether the object is locked into it. The authors also argue it is not a fragment of Haumea, because its color is redder than that of Haumea’s known collisional family.

Sources: NASA (Pluto, Eris, Haumea, Makemake); Minor Planet Center MPEC 2026-U85 and Holman et al. preprint (2026 ED138). Diameter for 2026 ED138 depends on its unmeasured surface reflectivity.

Why the find matters

The discovery offers an early look at what Rubin can do. The observatory began its 10-year Legacy Survey of Space and Time in June 2026 and will repeatedly image the entire southern sky. Before the survey formally started, its Solar System team had already submitted more than 11,000 new asteroids to the Minor Planet Center, including about 380 objects beyond Neptune, according to the observatory.

The researchers calculate that roughly three additional objects of this brilliance probably still await detection beyond the plane of the Milky Way, noting that the actual count is likely greater because the densely packed area close to the galactic plane has never been explored to this depth.

Michele Bannister, a planetary astronomer at the University of Canterbury in New Zealand who remained independent of the research team, described the body to CNN as “scientifically really cool,” highlighting its substantial dimensions alongside its position past Neptune.

What happens next

Additional data is required to verify the dimensions and nature of the object. Investigators advocate for thermal infrared observations—capable of distinguishing scale from albedo—alongside tracking the body as it transits background stars, a phenomenon known as stellar occultation. Analyses of the spectrum might indicate the presence of water ice or solid methane on its exterior, whereas fluctuations in luminosity over time could determine its sphericity. Holman mentioned to CNN that establishing the exact measurements might require a couple of months.

Much of the detailed analysis comes from a preprint that has not yet been peer reviewed. A separate preprint, on which Holman is a co-author, reports finding the object in archival images from NASA’s TESS satellite going back to 2019, independently supporting the calculated orbit.

The International Astronomical Union, which classifies such bodies, currently recognizes five dwarf planets: Ceres, Pluto, Eris, Haumea and Makemake. For now, 2026 ED138 remains a candidate.