A large object hidden against the crowded stars of the Milky Way could add another member to the solar system’s small family of recognized dwarf planets. Researchers using NSF-DOE Vera C. Rubin Observatory announced the candidate, provisionally named 2026 ED138, on Oct. 27.
The observatory’s announcement estimates that the object is 600 to 1,500 kilometers across. It lies beyond Neptune, more than 40 times the average Earth-Sun distance from the Sun. Its size and classification still need follow-up measurements. Scientists have found an object worth investigating, not a newly confirmed ninth planet.
A moving point in a crowded sky
Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian identified the object Oct. 13 while testing search software developed with colleague Kevin Napier. The program links detections that appear to belong to the same moving body.
According to Rubin, the observatory recorded ED138 30 times on 20 nights between March 4 and July 11. Its location near the Milky Way’s crowded plane helped it escape earlier detection. Infrared observations are needed to narrow the size estimate.
Why the size is still uncertain
A bright point does not give astronomers a ruler. In an interview with GBH, Holman explained that a smaller object with a very reflective surface could produce the same amount of observed light as a bigger, darker object. That leaves a wide range of possible diameters until researchers obtain more information.
GBH also interviewed Napier, who described the search as linking possible moving objects until enough observations allow an orbit to be estimated. Those are separate questions: researchers can work out a body’s path before they know its exact physical size.
That is why the word “candidate” matters in this announcement. The discovery can be real while the final description of the object remains unsettled. A future measurement could narrow the range without changing the fact that researchers have tracked the same distant body.
A trip around the Sun takes centuries
The Minor Planet Center’s Oct. 27 discovery circular lists an orbital period of about 281 years. Its published orbit is tilted about 24.5 degrees relative to the reference plane used for solar-system orbits. The path is therefore not a simple extension of a flat classroom drawing of the planets.
The circular includes observations from both Rubin’s Simonyi Survey Telescope and the Dark Energy Camera at Cerro Tololo in Chile. Its oldest listed measurements are from Feb. 25, 2016. Those older records give astronomers a much longer stretch of the object’s motion to examine than this year’s images alone.
The listed orbit has a semimajor axis of about 43 astronomical units. That describes the size of the orbit, not the object’s exact distance on every date. An object following an elongated path moves nearer to and farther from the Sun as it travels. Keeping those measurements separate helps avoid misleading comparisons.
How an old image becomes new evidence
Finding an object in images taken before its discovery is called precovery. The Asteroid Institute’s ADAM Precovery service starts with an estimated orbit, calculates where the object could have appeared and checks archived survey exposures that covered those positions.
That is more demanding than searching an image for a familiar name. The software has to allow for uncertainty in the predicted position and compare possible detections with the expected motion. Its results include information that helps researchers check whether a proposed match makes sense.
The service cautions that candidate detections require expert review before submission to the Minor Planet Center. An automated match is a lead to examine, not a substitute for that review. The value of an archive grows when a new discovery tells researchers where to look again.
What makes something a dwarf planet?
Under the definition adopted by the International Astronomical Union in 2006, a dwarf planet orbits the Sun, has enough mass for its gravity to make it nearly round, has not cleared its orbital neighborhood and is not a moon. NASA’s Jet Propulsion Laboratory explains that gravitational dominance of the orbital neighborhood separates a planet from a dwarf planet.
The category is therefore about more than being a small planet. An object must meet specific physical and orbital conditions. Identifying a distant point and measuring its motion are important steps, but they do not by themselves establish every part of that definition.
NASA lists five officially recognized dwarf planets: Ceres, Pluto, Eris, Haumea and Makemake. That list also shows why a candidate announcement should not be treated as an immediate change to the familiar count of eight planets. The two classifications describe different kinds of objects.
Why the outer solar system matters
The region beyond Neptune is not empty. NASA describes the Kuiper Belt as a broad, thick region containing icy remnants of the solar system’s early history. It includes small bodies and much larger worlds such as Pluto.
Those objects are useful because their distribution and motion carry clues about how the solar system developed. The giant planets helped shape the region, moving smaller bodies into different paths. Adding an accurately tracked object improves the inventory researchers use to test that history.
Distance also makes the work difficult. A small, cold world far from the Sun is not displayed as a detailed globe in a discovery exposure. Astronomers often begin with points of light and build an explanation through repeated measurements. The official observation composite accompanying this story shows that process more honestly than an imagined close-up of an unknown surface would.
A survey built to keep looking
Rubin’s 10-year Legacy Survey of Space and Time began in June 2026. The observatory plans to revisit each area of its survey sky roughly 800 times over the decade. Repetition allows researchers to identify motion and change that a single photograph cannot reveal.
Taking an inventory of the solar system is one of the survey’s four main goals. The others concern dark matter and dark energy, the Milky Way, and changes in the sky. The same observing program can therefore support several kinds of research rather than serving only one target.
For readers following ED138, the next useful milestone is a better measurement of the object itself. Rubin’s official orbital animation offers a way to explore its path while that work continues. For now, the discovery adds a promising candidate and a longer observational record to the map of our solar system, with its final classification still open.
