Boffin's software found a possible dwarf planet - a result so rare he thought it must be a bug
A scientist working on data gathered by the Rubin Observatory has found a strong candidate for our solar system's sixth known dwarf planet.
The Observatory is run by the USA's National Science Foundation and Department of Energy, who built it on a remote hilltop in Chile and equipped it with the 3200-megapixel Legacy Survey of Space and Time (LSST) Camera, which came online in June 2026.
As explained in a Tuesday announcement, Rubin Operations team member and project contributor Matthew Holman, of the Center for Astrophysics (CfA) at Harvard & Smithsonian, used search code he developed with CfA colleague Kevin Napier to analyze data captured before LSST operations commenced.
The Observatory says Holman's and Napier's code is built to look at billions of detections of moving objects, compare them, and find the detections that correspond to the same object."
On October 13, the code spotted an object so bright and distant that it is likely unusually large.
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."
Other astroboffins sprang into action, dug into other records, and soon found ten years' worth of relevant data.
Once they all checked their sums, the Minor Planet Center reviewed their work and announced the discovery of the romantically named 2026 ED138 and published lots of data about its orbit, which is at around 46 astronomical units (au). A single au is the distance from Earth to the Sun, and Pluto's orbit averages 40 au.
Rubin Observatory revealed its part in the effort and declared that 2026 ED138 is a good candidate for certification as a dwarf planet.
The 2026 International Astronomical Union defines a dwarf planet as an object in orbit around the Sun that is large enough to pull itself into a nearly round shape but has not been able to clear its orbit of debris."
The Union's definition of a full planet includes having an important influence on the orbital stability of the other objects in its neighbourhood." Dwarf planets whirl about the solar system in relatively close company with many rocky and/or icy objects that orbit under the influence of other bodies, not the nearby dwarf planet.
Rubin boffins think ED138 is between 600 and 1500 kilometers across. If the lower number is correct, it's the smallest dwarf planet yet observed. Should the larger number be correct, it would be smaller than Pluto, Eris, and Haumea, but larger than Makemake and Ceres.
Scientists at the observatory think ED138 has evaded detection to date because it was found in a region of sky close to the plane of the Milky Way - meaning observers had to pick it out from light cast by billions of stars.
Rubin Observatory's regular and continuous monitoring of the entire Southern Hemisphere sky, combined with powerful algorithms that sift through millions to billions of points of light to identify those corresponding to small moving objects, enabled the team to spot ED138 within the crowded field," the facility boasted. (R)