Rubin Observatory finds its first possible dwarf planet, 2026 ED138, out beyond Pluto

The Vera C. Rubin Observatory in Chile has found its first dwarf planet candidate, an object called 2026 ED138 that orbits the Sun beyond Pluto, the observatory said on 27 October. Its size is estimated at 600 to 1,500 km across, and the discovery was announced by the Minor Planet Center of the International Astronomical Union (IAU).

Hidden against the Milky Way

ED138 lies about 46 astronomical units from the Sun, 46 times Earth's distance, while Pluto averages about 40. Its orbit is tilted almost 25 degrees from the plane of the solar system, which suggests Neptune's gravity pushed it off its original path. It had gone unnoticed because it sits in a crowded part of the sky near the plane of the Milky Way, where it blended in with background stars.

Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian spotted it on the evening of 13 October, running Rubin data through search software he developed with colleague Kevin Napier. Mario Jurić of the University of Washington, Rubin's lead scientist for the solar system, then found 20 more sightings in Rubin's catalogs going back to 4 March. In all, Rubin's telescope recorded it 30 times on 20 nights between March and July 2026, during testing before its 10-year sky survey began in June.

“Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong.” — Matthew Holman, Center for Astrophysics | Harvard & Smithsonian

Older images trace a decade of its orbit

Once an object is known, astronomers can search older images for it. Using the Asteroid Institute's precovery service, the team found ED138 in images from the Dark Energy Camera dating back to 2016, which let them trace more than 10 years of its orbit. “Once we identify a new object, we can search back months, and sometimes even decades,” Jurić said.

Five recognized dwarf planets, and maybe a sixth

A dwarf planet orbits the Sun and is nearly round but has not cleared its orbit of other objects, according to NASA, which follows the IAU's definition. The IAU recognizes five: Ceres, Pluto, Haumea, Makemake and Eris, and says there may be more than a hundred still to be found.

ED138 is not on that list yet. Its size estimate is based on its distance and brightness and on an assumption about how much light its surface reflects, so infrared observations are needed to confirm how big it is and whether it is round enough to qualify. Rubin will image the whole southern sky again and again for 10 years, which should turn up more objects like it. “Rubin Observatory's super power is the ability to clearly capture movement and change,” said Edward Ajhar, program director for Rubin at the US National Science Foundation. Other recent telescope stories are collected under space exploration.