Astronomers Spot Possible New Dwarf Planet Beyond Pluto, Provisionally Named ED138

Earlier this year, Matthew Holman sat with the results of a computer search he had helped build, one that sifts through billions of detections of moving objects in data from the Vera C. Rubin Observatory. Something unexpected turned up. Holman, a Smithsonian senior astrophysicist and Harvard astronomy lecturer, says his first reaction was "excited disbelief," and his next move was to check whether it was a false positive or a fault in the program.

It was neither. The object, provisionally named ED138, appears to be a candidate dwarf planet beyond the orbit of Pluto. If later observations bear it out, it would join the five dwarf planets the International Astronomical Union currently recognises: Ceres, Pluto, Haumea, Makemake and Eris.

The tool that found it is new. Rubin sits on a 2682-metre mountain in Chile and houses the world's largest digital camera. It began operating in 2025 and can image the entire sky every three days. Holman, who is also on the Rubin Operations team, worked to confirm the find with 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 pair then traced the object's orbit back as far as 10 years.

The numbers so far are estimates. ED138's diameter is put at between 600 and 1500 kilometres, which would make it bigger than Ceres, the smallest recognised dwarf planet. It orbits the sun at about 46 astronomical units, each unit being the average distance between Earth and the sun. That places it beyond Pluto and makes it a trans-Neptunian object, one that orbits farther out than Neptune. It cannot be seen with the naked eye, though Holman says it is bright enough that "really capable amateurs" could spot it if they knew where to look.

Holman offered two reasons it went unnoticed. It lies in the southern hemisphere, which rules out many telescopes, and fewer instruments survey the southern sky anyway. It also sits along the plane of our galaxy; for the past 20 years, from our vantage point, it has been looping across the Milky Way. People rarely search there, Holman said, because the sheer number of stars makes moving objects hard to pick out.

He is careful about the label. "At this point, it's still a candidate," he said. Scientists need to know how physically big it is, which he expects to take a few months. The IAU's 2006 definition requires a dwarf planet to orbit the sun and be nearly round, without having cleared its orbit of debris. That same decision demoted Pluto and took the solar system from nine planets to eight. If ED138 qualifies, it will first get a permanent number, then a name, though the process may be slow and it is unclear who will choose the name. Holman says people have already emailed suggestions, some of them already taken. He is not counting on having a say.

Outside voices see wider meaning. Megan Schwamb of Queen's University Belfast, who is on the Rubin Solar System Operations team, said Rubin is "just doing its job really well, inventorying the Solar System to new depths." She was surprised it found such an object so early in commissioning observations, saying it "was not on my bingo card for this year." Holman thinks there may be a handful more objects of this size, and that Rubin should eventually find them.

Michele Bannister of the University of Canterbury, who was not involved, called the object scientifically exciting: a large trans-Neptunian object that is still fainter than the naked eye can see. Its size, she said, is just within the range where we can begin to think of it as a world. Other telescopes will now follow up. Whether it has something like Pluto's nitrogen glacier is unknown; for now, Bannister said, all we know is that it is big, and the rest is speculation. The next few months of size measurements will decide whether ED138 earns its place.