Astronomer Spots a Possible Dwarf Planet Roughly 1,000 Kilometers Across

Astronomers have found a large icy object beyond Pluto that could become the sixth officially recognized dwarf planet, according to ScienceDaily and the University of Washington. The object, temporarily named 2026 ED138, turned up in observations from the Simonyi Survey Telescope at the NSF-DOE Vera C. Rubin Observatory.

Its status is not settled. Researchers say they need more observations to pin down its size and shape before the International Astronomical Union could consider it alongside Pluto, Ceres, Eris, Haumea and Makemake.

Who found it, and how?

Matthew Holman, a senior astrophysicist at the Harvard & Smithsonian Center for Astrophysics, made the initial discovery on the evening of Oct. 13. He was running search software he developed with his colleague Kevin Napier on Rubin data gathered earlier in 2026. The program compares billions of detections and looks for points of light that move together in ways that suggest a single object.

Holman said his first reaction was disbelief, and that he immediately began checking whether he or the code had made a mistake.

How was the discovery checked?

Mario Jurić, a University of Washington astronomy professor who leads Rubin's solar system team, searched the observatory's stored data and found 20 additional detections of the object, some from as early as March 2026. The team then went through records from other surveys and found images of the object dating back to 2016. Together, the observations let them reconstruct more than a decade of its orbital motion.

How big is it, and where is it?

The object currently sits about 46 astronomical units from the Sun. One astronomical unit is the average distance between Earth and the Sun; Pluto's average distance is about 40. Astronomers estimate its width from its brightness, its distance and how much sunlight its surface reflects. By that method it could be anywhere from 600 to 1,500 kilometers across.

Jurić put it at roughly 1,000 kilometers and called a find of that size something that happens perhaps “once in a generation.” He said it is the largest trans-Neptunian object found in about 20 years.

Does that make it a dwarf planet?

Not automatically. An object that orbits beyond Neptune is called a trans-Neptunian object, but a dwarf planet must also have enough gravity to pull itself into a nearly round shape. An icy body in the size range estimated for this one would generally be expected to do so, according to the researchers, but they say more observations are required to confirm its dimensions and its category.

Why did such a large object stay hidden?

The object lies near the plane of the Milky Way, where millions of stars create a crowded background. Against that glare, its faint motion was hard for earlier telescopes and surveys to pick out. Rubin can repeatedly observe the Southern Hemisphere sky, and algorithms can sift through enormous amounts of data for small shifts in position, which made the difference here.

Why does it matter?

For general readers, the main significance is that the list of known worlds in the solar system may be growing, and that a large body could sit unnoticed in plain view. For scientists, distant objects like this one are valuable because their sizes, compositions and orbits preserve clues about how the solar system formed, how the giant planets shifted position and how smaller bodies were redistributed.

Holman said Rubin will image the whole southern sky again and again for 10 years, so other objects hiding in places earlier surveys skipped are now within reach. The discovery adds to a growing set of solar system finds from Rubin, which began its Legacy Survey of Space and Time in June 2026.

Around Prague, based on reporting by ScienceDaily

Cover photo: Rubin Observatory/NOIRLab/NSF/AURA/B. Quint / Wikimedia Commons, CC BY 4.0, cropped

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