Jens Asbjørn Bøgen
Two decades after Pluto was reclassified, astronomers keep finding new worlds in the solar system’s dark outer reaches.
The newest is big enough that gravity has likely pulled it into a sphere, and it has been hiding against the busiest patch of the night sky. It also happens to be the first major find from a telescope purpose-built to catch exactly this sort of thing.
The International Astronomical Union’s Minor Planet Center and the NSF-DOE Vera C. Rubin Observatory jointly announced the object on October 27, under the provisional designation 2026 ED138. Early measurements put its diameter between 600 and 1,500 kilometers, placing it among the largest trans-Neptunian objects on record. It currently sits about 46 astronomical units from the Sun, farther than Pluto’s roughly 40 AU average.
Matthew Holman, of the Center for Astrophysics | Harvard & Smithsonian, spotted ED138 on the evening of October 13 while running Rubin images through a search algorithm he developed with his colleague Kevin Napier.
“My first reaction was one of excited disbelief. Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong,” Holman said in the Rubin announcement.
Hiding against the Milky Way
ED138 had slipped past earlier surveys for two reasons. Its orbit tilts nearly 25 degrees off the plane the planets travel on, keeping it out of the strips of sky most asteroid hunters scan.
And when Rubin caught it, the object was passing in front of the Milky Way, a crowded region filled with millions of stars that tend to drown out faint, slow-moving dots.
After the first detection, Mario Jurić, the University of Washington astronomer who leads Rubin’s solar system work, searched the archive and found 20 more observations, bringing the total to 30 across 20 nights between March 4 and July 11. Older Dark Energy Camera images extended the orbital record to 2016.
A once-a-generation find
If confirmed, ED138 would join Pluto, Ceres, Eris, Haumea, and Makemake as the sixth body recognized as a dwarf planet by the International Astronomical Union.
“Finding an object this big, roughly 1,000 kilometers across, is something that happens maybe once in a generation,” Jurić said in the University of Washington’s announcement.
Over its ten-year Legacy Survey of Space and Time, Rubin’s Simonyi Survey Telescope will image the southern sky repeatedly, with algorithms built to pick out moving objects among billions of stellar sources. Phys.org notes that larger trans-Neptunian worlds hiding in the sky patches earlier surveys skipped should now be within reach.
