Astronomers Detect Possible Dwarf Planet Beyond Pluto Using Rubin Observatory

Astronomers analyzing observations from the Vera C. Rubin Observatory in Chile have detected a massive, icy celestial body in the outer reaches of the solar system that could qualify as a new dwarf planet. Designated 2026 ED138 by the International Astronomical Union's Minor Planet Center, the trans-Neptunian object circles the sun well beyond the orbit of Pluto. The finding, presented at a scientific meeting in Spokane, Washington, represents one of the most prominent planetary candidates spotted in the Kuiper Belt in recent years.

Preliminary estimates place the diameter of 2026 ED138 between 370 and 930 miles, or roughly 600 to 1,500 kilometers, putting it in the same general size range as several recognized dwarf planets. The object is located approximately 46 astronomical units from the sun, with one astronomical unit equaling the average distance between Earth and the sun. Pluto, by comparison, orbits at an average distance of about 40 astronomical units. If officially confirmed, 2026 ED138 would join Ceres, Pluto, Haumea, Makemake, and Eris as the sixth dwarf planet recognized by the International Astronomical Union.

Matthew Holman, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics, identified the candidate while analyzing data gathered by the observatory alongside software developed with astronomer Kevin Napier. Rubin's Simonyi Survey Telescope logged sightings of the object around 30 times between March 4 and July 11. Holman said his initial reaction was "one of excited disbelief," prompting him to immediately double-check his calculations and code because discoveries of this scale are so rare.

Despite its substantial size and notable brightness, which experts say could allow it to be observed with relatively modest telescopes, 2026 ED138 went unnoticed for decades. Researchers explained that the world managed to conceal itself against the dense backdrop of stars in the Milky Way's galactic disk, where concentrated starlight makes tracking movement difficult. In addition, its 281-year orbit is tilted roughly 24.5 degrees relative to the ecliptic plane, keeping it away from regions historically prioritized by conventional planetary surveys.

Following the initial detection, scientists quickly reconstructed the object's path across an entire decade. Using the Asteroid Institute's ADAM Precovery tool to scan astronomical archives, the research team matched the object to 2016 imagery taken by the Dark Energy Camera at another facility in Chile. Mario Jurić, the lead solar system scientist for Rubin at the University of Washington, noted that the ability to precover objects across historical archives has transformed how rapidly astronomers can calculate precise orbits and map the distant fringes of the solar system.

To secure formal dwarf-planet classification, researchers must confirm that 2026 ED138 meets strict physical criteria. While full-fledged planets must clear other large objects from their orbital path, dwarf planets share their orbital zone with other bodies but must possess sufficient gravity to pull themselves into a rounded shape, a state termed hydrostatic equilibrium. Although scientists anticipate that an icy world of this size would achieve roundness, its shape and mass remain unverified.

Resolving its status will require targeted follow-up observations through infrared and submillimeter telescopes, such as the Atacama Large Millimeter/submillimeter Array in Chile. Holman explained that optical telescopes like Rubin only detect sunlight reflected from the surface, making it difficult to differentiate between a smaller, highly reflective body and a larger, darker one. Measuring the candidate at longer wavelengths will record thermal emissions directly from the object, allowing scientists to pinpoint its actual diameter and surface properties.

Funded by the U.S. National Science Foundation and the U.S. Department of Energy, the Rubin Observatory operates atop Cerro Pachón and features the largest digital camera ever constructed for astronomy. With plans to repeatedly scan the entire southern sky over the next decade, researchers view the detection of 2026 ED138 as an early indicator of what Rubin can accomplish, offering fresh opportunities to study ancient planetary building blocks from the dawn of the solar system.