Rubin Observatory found a possible sixth dwarf planet up to 1,500 km wide

The Vera C. Rubin Observatory has found an icy world beyond Pluto that may be big enough to count as a dwarf planet. First estimates put it somewhere between six hundred and fifteen hundred kilometres across, which would rank it among the largest bodies ever discovered past Neptune.

If it passes the tests still ahead, the solar system would gain a sixth recognised dwarf planet, joining Pluto, Eris, Haumea, Makemake and Ceres. The find also shows what Rubin was built for. The object had been in plain sight for years, lost in a crowd of stars, until software designed to catch faint moving dots picked it out.

The object carries the provisional designation 2026 ED138, and the International Astronomical Union’s Minor Planet Center announced it. Its discoverer, Matthew Holman of the Harvard & Smithsonian Center for Astrophysics, presented it with Mario Jurić of the University of Washington, who leads Rubin’s solar system science, at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Spokane, Washington.

“My first reaction was one of excited disbelief,” Holman said. “Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong.”

Jurić put the find in generational terms. “Finding an object this big, roughly 1,000 kilometers across, is something that happens maybe once in a generation,” he told GeekWire. “It’s the largest trans-Neptunian object found in about 20 years, and Rubin turned it up while we were still commissioning the telescope.”

How Rubin pulled a slow dot out of a crowd of stars

Objects beyond Neptune crawl across the sky so slowly that a single image cannot separate them from stars. Holman found ED138 by running Rubin data through search code he wrote with his Center for Astrophysics colleague Kevin Napier. The code compares billions of detections of moving points of light and links the ones that belong to the same object from night to night.

Once ED138 stood out for its brightness and distance, Jurić went back through Rubin’s catalogues and images and found 20 more sightings. In total, Rubin’s Simonyi Survey Telescope in Chile had recorded the object 30 times over 20 nights. The Asteroid Institute’s ADAM Precovery service then found it in archival frames from the Dark Energy Camera on the Victor M. Blanco 4-metre telescope at Cerro Tololo, stretching the record back about a decade.

Why did nobody catch it earlier? ED138 sits close to the plane of the Milky Way, a strip of sky packed with millions of stars where a faint, slow object easily disappears among background sources. Rubin images the entire southern sky over and over, and its software sifts millions to billions of points of light, so a dot that drifts against that crowd eventually gives itself away. Jurić said the team can now look backward as well as forward: “once we identify a new object, we can search back months, and sometimes even decades, finding it in images where nobody knew to look.”

Where it orbits

ED138 is now about 46 astronomical units from the Sun, roughly 6.9 billion kilometres, or 46 times the distance between Earth and the Sun. Sunlight takes more than six hours to reach it. Pluto’s average distance is 40 astronomical units.

A preliminary orbit from NASA’s Jet Propulsion Laboratory puts ED138’s average distance at about 43 astronomical units, swinging between roughly 37 and 49. At that distance, one lap around the Sun takes about 280 years. Its path never comes inside the orbit of Neptune, which sits at about 30 astronomical units.

The orbit is tilted almost 25 degrees from the plane of the planets. That tilt makes ED138 what astronomers call a hot classical object, and it suggests that Neptune’s gravity knocked it off its original course at some point in the solar system’s past.

How big is it, really?

Nobody has measured ED138’s size. The team worked it out from three ingredients: its distance, how bright it looks, and an estimate of how much light its surface reflects. The last one carries most of the uncertainty. A dark surface would need to be large to shine as brightly as ED138 does, while fresh, reflective ice would need much less area. That is why the estimate runs from 600 to 1,500 kilometres.

The two ends of that range describe quite different worlds. At 600 kilometres, ED138 would be smaller than Ceres, the dwarf planet in the asteroid belt, which measures about 950 kilometres. At 1,500 kilometres it would rival Makemake, at about 1,430 kilometres, and Haumea, at about 1,540. Jurić’s working figure of roughly 1,000 kilometres puts it in the class of Quaoar, about 1,100 kilometres across. Pluto and Eris, at about 2,380 and 2,330 kilometres, stay well ahead.

The “largest in about 20 years” comparison also holds up. Gonggong, about 1,230 kilometres wide, was found in 2007. Most of the big names out there, including Quaoar, Sedna, Haumea, Orcus, Makemake and Eris, were found between 2002 and 2005.

What it doesn’t settle

ED138 is a dwarf planet candidate, not a dwarf planet. Under the definition the International Astronomical Union adopted in 2006, a dwarf planet must orbit the Sun and have enough gravity to pull itself into a round shape, a state known as hydrostatic equilibrium, without having cleared its orbital neighbourhood. Icy bodies of the size estimated for ED138 are expected to be round, but nobody has yet shown that this one is.

Its mass is also unknown. For a world this far away, astronomers usually weigh it by timing the orbit of a moon, and the announcement mentions none. The orbit itself is still preliminary and will sharpen as more observations come in.

Even a round ED138 may not get onto the official list quickly. “The official list hasn’t moved since 2008,” Jurić told GeekWire, adding that he was not sure whether any active process reviews candidates. Several bodies of similar size, Quaoar and Gonggong among them, are widely treated as dwarf planets in research papers but still sit outside the union’s list of five.

Common questions about 2026 ED138

What is 2026 ED138?

It is an icy object that orbits the Sun beyond Neptune, currently about 46 times farther from the Sun than Earth. The Vera C. Rubin Observatory found it, and with an estimated width of 600 to 1,500 kilometres it is one of the largest trans-Neptunian objects known.

Is 2026 ED138 a dwarf planet?

Not officially. It is a candidate. To qualify it has to be shown to be round, and that needs follow-up observations such as stellar occultations and thermal measurements. Even then, the International Astronomical Union would have to add it to its list.

How many dwarf planets are there?

The International Astronomical Union recognises five: Ceres in the asteroid belt, and Pluto, Eris, Haumea and Makemake beyond Neptune. Astronomers consider several other large bodies, such as Quaoar, Gonggong and Sedna, likely dwarf planets, but they are not on the official list.

How did the Rubin Observatory find it?

Software written by Matthew Holman and Kevin Napier linked repeated detections of the same faint moving point across Rubin’s images. Further searches turned up 30 Rubin sightings and older Dark Energy Camera images that extend its track back about a decade.

What comes next

Holman found ED138 on the evening of 13 October 2026, in data Rubin had collected between 4 March and 11 July, and the Minor Planet Center announced it on 27 October. Archival images go back to 2016. Rubin’s ten-year Legacy Survey of Space and Time formally began in June 2026. In April, before the survey started, the team had already sent the Minor Planet Center more than 11,000 new asteroids, including roughly 380 new trans-Neptunian objects.

The next job is to prove ED138 is round. Jurić told GeekWire the most direct route is a stellar occultation, the moment the object passes in front of a background star and briefly blocks its light, and that the odds of catching one are good “given it’s in a dense stellar field.” Thermal measurements, modelling and light curves would follow, and infrared observations are needed to narrow the size. The same occultation technique gave Eris its precise size, and now that ED138’s orbit is known, observers anywhere can plan for it. The public can follow the object’s path in Rubin’s Orbitviewer app.

“If confirmed as a dwarf planet, ED138 would join a very small family of worlds,” Holman said. “Rubin will image the whole southern sky again and again for ten years, so any others hiding in the places earlier surveys skipped are now within reach.”

Tags: astronomy, PLUTO, solar system, trans-neptunian objects, 2026 ED138, dwarf planets