Rubin Observatory Spots Possible New Dwarf Planet Candidate ED138 Beyond Pluto

Sometime in 2026, a computer program running through Rubin Observatory data turned up something that made Matthew Holman stop. The Smithsonian senior astrophysicist, who also lectures in astronomy at Harvard, had set the software, which he co-developed, to comb through billions of detections of moving objects. One of them looked like a dwarf planet candidate beyond Pluto. His first reaction, he said, was "excited disbelief," and his first move was to check whether it was a false positive or a bug.

The object has been provisionally named ED138. It was found in data from the Vera C. Rubin Observatory, which sits on a 2,682-meter (8,800-foot) mountain in Chile and houses the world's largest digital camera. The telescope began operating in 2025 and can image the entire sky every three days.

To test the find, Holman, a member of the Rubin Operations team, turned to colleague Mario Jurić, the Rubin Solar System lead scientist and a University of Washington astronomy professor. Jurić searched Rubin's catalog and found 20 more observations of ED138, going back to March 2026. Using other sources, the researchers then traced the object's orbit as far back as 10 years.

Even so, Holman is careful about the label. "At this point, it's still a candidate," he said. "We really need to know how physically big it is, and that will take a few months, probably." Current estimates put the diameter between 600 and 1,500 kilometers (370 to 930 miles). That would make it larger than Ceres, the smallest of the five dwarf planets the International Astronomical Union recognizes: Ceres, Pluto, Haumea, Makemake and Eris. ED138 would be the sixth.

It orbits the sun at about 46 astronomical units, each unit being the average Earth-sun distance. That places it beyond Pluto and makes it a trans-Neptunian object, one that circles the sun farther out than Neptune. It cannot be seen with the naked eye, but Holman said it is bright enough that "really capable amateurs" who know where to look can see it.

Holman gave two reasons it escaped notice. It lies in the southern sky, which rules out many telescopes and leaves fewer searching there. It also sits in the plane of the galaxy; for the last 20 years it has been looping across the Milky Way from our vantage point, and people rarely hunt for moving objects among so many stars.

Holman thinks there may be "a handful" more objects of similar size, and expects Rubin to find them. Megan Schwamb, a reader in astrophysics at Queen's University Belfast and a member of the Rubin Solar System Operations team, said in an email that the observatory is "just doing its job really well, inventorying the Solar System to new depths." She was surprised to see such an object so early in commissioning observations: "This was not on my bingo card for this year."

The category itself dates to 2006, when the IAU defined a planet as an object that orbits the sun, is mostly round, and has cleared its orbit of similarly sized objects. Pluto failed the last test, and the count fell from nine planets to eight. Pluto and similar bodies became dwarf planets. If ED138 is confirmed, it will get a permanent number and then a name, though the process could take time and it is unclear who will choose. Holman said people have already emailed suggestions, some already used by other objects, and he isn't counting on having a say.

Michele Bannister, an associate professor at the University of Canterbury in New Zealand who was not involved, called it "scientifically really cool" because it is large yet still far fainter than the naked eye can see. She said its size is just in the range where it can be thought of as a world, and other telescopes will now follow up. Whether it has something like Pluto's nitrogen glacier is unknown; for now, she said, all that is known is that it is big. The next step is measuring that size, which should take a few months.