On 27 October 2026, astronomers announced a possible new member of the solar system's most exclusive club: the 2026 ED138 dwarf planet candidate, a large icy world orbiting far beyond Neptune that may become the sixth recognised dwarf planet. The object was found by Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian, who spotted it on 13 October 2026 while running search code on data from the Vera C. Rubin Observatory in Chile. If confirmed, the 2026 ED138 dwarf planet would join Pluto, Ceres, Eris, Haumea and Makemake, the only five worlds officially recognised as dwarf planets today. For now, though, it remains a candidate, not a confirmed dwarf planet, and the story of 2026 ED138 is as much about how astronomers measure a world they have barely seen as it is about the discovery itself. This explainer covers what the object is, why its size is still a wide estimate, why it hid for so long, how its 281-year orbit works, and what must happen before it can be confirmed.
What Is the 2026 ED138 Dwarf Planet Candidate?
The 2026 ED138 dwarf planet candidate is a trans-Neptunian object, which means it orbits the Sun beyond the planet Neptune, in the distant region of the solar system where Pluto and the other dwarf planets live. Its name, 2026 ED138, is a provisional designation given to the object when it was found. Holman discovered it using search code built with Kevin Napier, sifting through observations from the Rubin Observatory, and the discovery was announced by the International Astronomical Union's Minor Planet Center on 27 October 2026. The same day, the Rubin Observatory published its official release describing the find as its first dwarf planet candidate, and Holman presented the discovery with Mario Jurić of the University of Washington, the Rubin Observatory's solar-system science lead, at the American Astronomical Society's Division for Planetary Sciences meeting. The object sits about 46 astronomical units from the Sun, where one astronomical unit is the average distance between the Earth and the Sun, while Pluto averages about 40 astronomical units, so 2026 ED138 orbits farther out on average than Pluto. That average distance is the key fact behind calling the 2026 ED138 dwarf planet a world beyond Pluto: not that it has overtaken Pluto in a race, but that its home is farther from the Sun than Pluto's average orbit.
How Big Is the 2026 ED138 Dwarf Planet? Why the Range Is 600 to 1,500 km
Astronomers estimate that the 2026 ED138 dwarf planet candidate is between 600 and 1,500 kilometres across, or about 370 to 930 miles, and that wide range is not a mistake. It comes from the way size is measured for an object this far away. Telescopes can measure how bright the object looks and how far away it is, but brightness alone does not give size, because a small, bright, icy surface reflects far more sunlight than a large, dark one. The missing piece is the object's reflectivity, known as its albedo, and until that is measured the size can only be given as a range. If the surface is dark, the object must be near the top of the range to look as bright as it does; if it is bright and icy, it could be near the bottom. That range matters because it decides where the object would rank. At the low end of 600 kilometres it would be the smallest dwarf planet yet recognised, while at the high end of 1,500 kilometres it would be larger than Makemake and Ceres, though still smaller than Pluto, Eris and Haumea. As Jurić put it, the object is roughly 1,000 kilometres across and a find of this kind comes "once in a generation". Only infrared observations, which measure the heat the object gives off rather than the sunlight it reflects, can pin down the 2026 ED138 dwarf planet candidate's reflectivity and its true size, and those observations are also what astronomers need before they can confirm it as a dwarf planet at all.
Why the 2026 ED138 Dwarf Planet Candidate Hid for So Long
The 2026 ED138 dwarf planet candidate hid for so long partly because of where it sits in the sky. It was found in a crowded region near the plane of the Milky Way, where thousands of background stars make it hard to pick out a faint, slow-moving speck of light. A distant icy world moves only slightly against the stars from night to night, so finding one means comparing images taken over weeks and months and looking for the single dot that shifts. That is what Holman's search code did with Rubin Observatory data collected between 4 March and 11 July 2026, gathering 30 observations on 20 nights. Astronomers then found the object in archival images going back to 2016, giving them more than ten years of positions to work out its orbit. According to the Rubin Observatory, it is the largest trans-Neptunian object found in about 20 years, and it is bright enough that well-equipped amateur astronomers could find it too. Modern sky surveys generate data on a scale that needs serious computing power to search, which is one reason investment in computing and artificial intelligence has become central to astronomy, as seen in the recent wave of AI financing in the space industry.
A 281-Year Orbit Tilted About 25 Degrees
The 2026 ED138 dwarf planet candidate takes about 281 years to complete a single orbit around the Sun, according to New Scientist, and that orbit is tilted about 25 degrees to the plane of the solar system. That tilt is part of why the object stayed hidden: most planets and many small bodies orbit close to the same flat plane, so surveys that concentrate on that plane can miss objects that spend much of their time above or below it. The long orbital period also means astronomers have watched only a tiny fraction of one full circuit, which is why the decade of archival images back to 2016 matters so much. Those older observations stretch the known arc of the orbit and let astronomers calculate its path with far more confidence than the few months of Rubin data alone would allow. At about 46 astronomical units from the Sun, the object is deep in the trans-Neptunian region, well beyond Neptune's orbit, and its slow, tilted journey around the Sun is typical of the distant icy bodies that have survived largely unchanged since the solar system formed.
What Would Confirm 2026 ED138 as a Dwarf Planet?
Confirming the 2026 ED138 dwarf planet candidate will take more than spotting it. Under the definition used by the International Astronomical Union, a dwarf planet must orbit the Sun, must be massive enough for its own gravity to pull it into a nearly round shape, must not have cleared its neighbourhood of other objects, and must not be a moon. The infrared observations now planned are the key next step: by measuring the heat the object radiates, astronomers can work out its reflectivity and its true size, and size is closely tied to whether gravity has made it round. Until those measurements are in, the 2026 ED138 dwarf planet candidate stays a candidate, and every careful report, including this one, must keep calling it one. Space agencies are also studying small distant worlds directly, which is how the science of these bodies advances beyond telescope images alone: Japan's MMX mission to the Martian moon Phobos will bring a sample of a small body back to Earth, the kind of hands-on evidence that remote observations cannot provide. For 2026 ED138, though, there is no mission on the way; confirmation will come from telescopes, patiently, over the coming months and years.
Where 2026 ED138 Would Rank Among the Dwarf Planets
If confirmed, the 2026 ED138 dwarf planet would become the sixth recognised dwarf planet, joining a small and famous group: Pluto, Ceres, Eris, Haumea and Makemake. Where it would sit in that ranking depends entirely on the size measurement that is still to come. At the high end of its 600 to 1,500 kilometre range it would be larger than Makemake and Ceres, placing it in the middle of the group, while at the low end it would be the smallest of the six. Either way, its discovery is a reminder of how much of the outer solar system remains unmapped. The Rubin Observatory has only just begun its great sky survey, and if a world this large could hide near the Milky Way until 2026, more are almost certainly waiting in the same crowded star fields. The 2026 ED138 dwarf planet candidate may or may not win its place among the six, but the search that found it has already shown that the solar system still holds surprises.