2026 ED138 Joins a Dwarf Planet Queue That Has Not Moved Since 2008

For eighteen years, the list has stopped at five. Makemake was named in July 2008. Haumea followed that September. Nothing has been added since, even as telescopes kept finding large icy bodies beyond Neptune. The announcement on October 27 of 2026 ED138 does not change that count. It adds a name to a waiting line that has been standing still longer than the candidate has been observed.

Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian found the object on October 13. He was running Rubin Observatory data from earlier in 2026 through a search code he developed with Kevin Napier, a program built to compare billions of detections of moving objects and pick out the ones that belong to the same body. His first reaction, he said, was “excited disbelief,” and he checked for a false positive or a coding error before anything else. Rubin had caught the object 30 times on 20 nights between March 4 and July 11, and archival images from the Dark Energy Camera traced it back to February 2016, giving the team more than ten years of orbit to work with. The International Astronomical Union’s Minor Planet Center announced the find on October 27, and Holman presented it with Mario Jurić of the University of Washington at the Division for Planetary Sciences meeting in Spokane.

The circular the Minor Planet Center published carries the orbit in full. 2026 ED138 circles the Sun every 281 years. Its orbit has a semi-major axis of 42.95 astronomical units, an eccentricity of 0.14 and an inclination of 24.47 degrees, coming no closer than 36.86 astronomical units and going no farther than about 49.04. It sits about 46 astronomical units out right now, which is why the coverage calls it a world beyond Pluto, but the full loop tells a narrower story. Pluto’s own orbit runs from 29.7 to 49.3 astronomical units over 248 years at an inclination of 17.2 degrees, so 2026 ED138 never travels farther from the Sun than Pluto does at its farthest point and never comes as close as Pluto does at its nearest, and its path is barely half as stretched out of shape. The Rubin team classifies it as a hot-classical trans-Neptunian object, tilted steeply enough that Neptune’s gravity probably knocked it out of its original orbit long ago.

The circular prints no diameter at all. It prints an absolute magnitude, H = 2.93, which is how bright the object would look from a standard distance, and brightness alone cannot fix a size. A small, reflective body throws back as much sunlight as a large, dark one, so the diameter has to be worked out across the range of surfaces the object might have. At a reflectivity of 5 percent, the kind of dark surface common out there, H = 2.93 gives a diameter near 1,550 kilometers. At 50 percent reflectivity, it gives roughly 490 kilometers. The Rubin team’s published range of 600 to 1,500 kilometers sits inside that bracket. The spread matters. A factor of about two and a half in diameter is a factor of roughly sixteen in volume, so the object could be a modest ice world or one of the largest bodies past Neptune, and the observations so far cannot say which. The low end also sits well below Ceres, the smallest recognized dwarf planet at about 939 kilometers, which means the claim that the newcomer would be bigger than Ceres holds only if the true size lands in the upper part of the range. Icy bodies this large are expected to be round under their own gravity, as the Rubin release notes, but expected is a prediction about a class, not a measurement of this body.

Under the definition the IAU adopted in 2006, a dwarf planet orbits the Sun, is massive enough for its gravity to pull it round, and has not cleared its orbit of other bodies. The naming procedure the IAU set in 2008 adds a practical filter on top of that definition: an unnamed trans-Neptunian object brighter than absolute magnitude +1, which corresponds to a minimum diameter of 838 kilometers even at perfect reflectivity, goes to a joint naming committee, and once named it is declared a dwarf planet. Haumea and Makemake are the only bodies to have gone through that track. 2026 ED138, at H = 2.93, is nearly two magnitudes fainter than the threshold, roughly six times fainter in brightness, so it cannot ride the automatic route. Its recognition would need a separate decision by the IAU, and the union has not made one of those for any object since 2008.

Gonggong, found in 2007, measures 1,230 kilometers across with an uncertainty of 50 kilometers, which puts it larger than Pluto’s moon Charon at 1,212 kilometers, and it is still not one of the five. Quaoar, found in 2002, measures close to 1,090 kilometers. Orcus, found in 2004, measures about 910 kilometers, and Sedna, found in 2003, measures roughly 900 kilometers, though its size carries a wide uncertainty. In 2008, the same year Haumea and Makemake were named, Tancredi and colleagues advised the IAU to accept Orcus, Sedna and Quaoar as dwarf planets, and the union did not take up the recommendation then and has not returned to it since. Gonggong was not yet known when that advice was written, and it has waited ever since under its own name, chosen in a 2019 public poll, without the category to match.

“At this point, it’s still a candidate,” Holman told CNN. “We really need to know how physically big it is, and that will take a few months, probably.” The measurement he points to uses heat rather than reflected light: at longer wavelengths, with an observatory like ALMA, astronomers can detect “the light emitted by the object itself,” which breaks the trade-off between size and reflectivity that visible light cannot resolve, and the Rubin release says infrared observations are needed to pin the size down. The object is bright enough in the meantime that Holman says really capable amateurs can see it if they know where to look, at an apparent magnitude near 20. Megan Schwamb of the Rubin Solar System Operations team, writing to CNN, said she was surprised the telescope found such an object so early in its commissioning observations, and Jurić called a find this large, roughly 1,000 kilometers across, something that happens maybe once in a generation, the largest trans-Neptunian object found in about 20 years. Holman also told CNN there might be a handful more objects the size of 2026 ED138 still to be found.

Rubin’s own survey simulation puts the ten-year catalogue at about 37,000 trans-Neptunian objects. The team’s April submission, made before the survey formally began, already contained roughly 380 new ones alongside more than 11,000 new asteroids.