ED138: A Large Dwarf Planet Candidate Discovered Beyond Neptune

How ED138 Was Found

ED138 was identified by Rubin Operations team members using a dedicated search code designed to comb billions of detections for moving objects. On the evening of 13 October 2026, Matthew Holman (Center for Astrophysics | Harvard & Smithsonian) ran this code and uncovered ED138. He immediately recognized that its brightness and distance indicated an unusually large object.

To confirm the signal, Holman and Mario Jurić (University of Washington) searched Rubin’s catalogs and images and found 20 additional observations, expanding the observational span. In total, Rubin’s Simonyi Survey Telescope observed ED138 30 times over 20 nights between March 4 and July 11, 2026. The team also recovered archival data from DECam, mounted on the Blanco telescope at CTIO, dating back to 2016. These archival images were provided by the ADAM Precovery service and allowed the orbit to be traced over more than a decade.

The distance of ED138 places it beyond Pluto, at about 46 astronomical units (au) from the Sun. For reference, Pluto orbits at roughly 40 au. This makes ED138 a trans-Neptunian object (TNO) and a prime candidate for hydrostatic-geology studies in the outer Solar System.

Rubin’s discovery was announced through the Minor Planet Center, and the results were presented at a press conference during the 58th Annual Division for Planetary Sciences (DPS) meeting in Spokane, Washington. The collaboration emphasized Rubin’s ability to both discover new objects and “precover” them—finding them again in past images to extend their orbital arcs.

Follow-Up Observations and Data Access

The ED138 data are being made available to Rubin data rights holders on the Rubin Science Platform, a web-based environment for accessing, exploring, and processing Rubin’s time-domain data. The public can also engage with the discovery through Orbitviewer, an interactive app that lets users explore ED138’s orbit in 3D and in real time.

Rubin’s multi-pronged approach—continuous, wide-field monitoring combined with deep, archival data—has already yielded a substantial yield. In April 2026, Rubin submitted observations of more than 11,000 new asteroids to the IAU Minor Planet Center, including roughly 380 new TNOs. This productivity underscores Rubin’s potential to expand the census of distant Solar System objects as the Legacy Survey of Space and Time proceeds.

Infrared observations are expected to be an important follow-up step to better constrain ED138’s size and surface properties and to help determine whether the object is in hydrostatic equilibrium. Possible facilities for such follow-up include the James Webb Space Telescope (JWST) and ground-based infrared observatories.

What ED138 Tells Us About the Outer Solar System

Preliminary estimates place ED138’s diameter in the broad range of roughly 600 to 1,500 kilometers. If ED138 is confirmed to have a hydrostatic shape, it would strengthen its case as a true dwarf planet, distinct from mere large Trans-Neptunian Objects.

ED138’s orbit is tilted by about 25 degrees relative to the plane of the Solar System. This places it in the hot-classical TNO category, a dynamical group believed to have been perturbed from their original orbits by past gravitational interactions with Neptune. Explaining this tilt helps researchers understand how the outer Solar System evolved and how Neptune’s gravity shaped the current distribution of icy bodies beyond its orbit.

In the broader context, the five officially recognized IAU dwarf planets are Pluto, Ceres, Eris, Haumea, and Makemake. Astronomers often discuss other large TNOs—Gonggong (formerly 2007 OR10), Quaoar, Sedna, Orcus, and Salacia—as dwarf-planet–class candidates, though the IAU’s formal recognition currently centers on the five named above. The landscape of potential dwarf planets continues to grow as surveys like Rubin push to larger sizes and greater orbital distances.

Context on Classification and What It Requires

A body earns dwarf-planet status when it is massive enough for its gravity to pull it into a round shape (hydrostatic equilibrium) but has not cleared its neighborhood of smaller bodies. This distinction—between gravitational dominance and an inability to clear the surrounding area—helps separate dwarfs from the eight classical planets. The exact size at which an object becomes round depends on its composition and history, so the lower bound for hydrostatic equilibrium is not a fixed value. For ED138, the current size estimate and ongoing follow-up observations will determine whether it truly satisfies that hydrostatic criterion.

Rubin’s discovery, including the use of precovery data from DECam and the ability to search for moving objects in crowded stellar fields, demonstrates a practical example of how precovery and data mining extend orbital coverage. This combination is central to refining orbits and enabling long-term dynamical studies of distant bodies.

The Team, Platform, and Access to Data

The discovery and analysis involved a collaboration among Rubin Solar System scientists, with key figures including Matthew Holman (CfA) and Mario Jurić (University of Washington) taking prominent roles in developing the search algorithms and presenting results at the DPS meeting.

ED138’s data are accessible via the Rubin Science Platform to data-rights holders, while Orbitviewer provides a public-facing visualization of the orbit. Rubin Observatory’s broader mission—carried out in collaboration with the U.S. National Science Foundation and the U.S. Department of Energy—will continue to deliver a vast time-domain data set for solar system science over the next decade.

Rubin’s early data began in preparation for the LSST, which formally began data collection in June 2026. In the run-up to that start, the team highlighted the volume of discoveries they were already making, including submitting more than 11,000 asteroid observations and hundreds of newly identified TNOs.

What’s Next: A Timeline of Follow-Up

The immediate next steps involve confirming whether ED138 is in hydrostatic equilibrium and refining its size and mass estimates through follow-up observations in the infrared and through additional astrometric monitoring. Infrared facilities such as JWST or capable ground-based infrared telescopes are potential options for constraining surface composition and thermal properties, which feed into diameter estimates and density calculations.

Over the coming months and years, Rubin will reimage the region, extend the asteroid and TNO census in the outer Solar System, and invite the wider community to participate in follow-up campaigns. As more data accumulate, the scientific community will be able to judge whether ED138 becomes a fully confirmed dwarf planet in the same class as Pluto, Ceres, Eris, Haumea, and Makemake.

Important Notes for Readers

2026 ED138 is currently a provisional designation. The IAU Minor Planet Center and Rubin’s science teams will determine whether it earns a formal name and, if so, whether it meets the hydrostatic-equilibrium criterion required for dwarf-planet status.

This article follows the latest Rubin press releases and DPS presentations, which emphasize the object’s large size, distant orbit, and tilted trajectory as key factors shaping its interpretation as a potential hot-classical TNO and a candidate for a future confirmed dwarf planet.

The discovery highlights Rubin’s ability to uncover distant, moving objects even in crowded star fields near the Milky Way’s plane, underscoring the importance of deep, wide-field surveys for building a complete census of the outer Solar System.