Rubin dwarf planet candidate found beyond Pluto

The Rubin dwarf planet candidate orbits the Sun beyond Pluto, and early analysis suggests it may be among the largest and brightest objects ever found in that distant region. The International Astronomical Union Minor Planet Center announced the discovery on October 27, after observations from the NSF-DOE Vera C. Rubin Observatory in Chile. The candidate carries the provisional designation 2026 ED138, and scientists say follow-up work will determine whether it earns a place on the short official list of dwarf planets.

Matthew Holman, a Rubin operations team member at the Center for Astrophysics at Harvard and Smithsonian, found the object while running data collected earlier in the year through search code he developed with colleague Kevin Napier. According to the observatory, the code sifts through billions of detections of moving objects and groups the detections that belong to the same body. Holman noticed the brightness and distance at once, and both pointed to an unusually large object. He then checked the result carefully before telling colleagues, because objects of this size are rare in the outer solar system.

Rubin Observatory is funded jointly by the United States National Science Foundation and the Department of Energy Office of Science. Edward Ajhar, the NSF program director for Rubin, said in a statement that the observatory can capture movement and change across vast numbers of objects and phenomena that people have not seen before. Kathy Turner, the Department of Energy program manager for Rubin, said the observatory can investigate a wide range of astrophysical questions, from dark energy to the discovery of this candidate.

Found in a crowded patch of sky

Finding the Rubin dwarf planet candidate took patience because it sits in a region of sky close to the plane of the Milky Way, where millions of stars crowd the view. According to the observatory, most telescopes would lose such a faint moving point among the background sources, and that crowding probably explains why earlier surveys missed it. Rubin watches the southern sky on a regular schedule, and its algorithms compare points of light across many images to pick out bodies that move against the stars.

The search turned up a long trail. Mario Juric, the Rubin solar system lead scientist at the University of Washington, searched Rubin catalogs and images and located twenty additional observations. In total, Rubin observed the candidate thirty times on twenty nights between March 4 and July 11. Archival images from the Dark Energy Camera, uncovered through the Asteroid Institute precovery service, extended the record back to 2016 and allowed the team to trace more than a decade of its orbit.

Distance helps explain why the object stayed hidden for so long. The candidate now sits about 46 astronomical units from the Sun, where one astronomical unit is the average distance between Earth and the Sun. Pluto, by comparison, averages 40 astronomical units from the Sun. The new object therefore counts as a trans-Neptunian object, a body that orbits the Sun at a greater distance than Neptune. Its orbit tilts almost 25 degrees from the plane of the solar system, a shape that suggests gravitational encounters with Neptune pushed it from an earlier path.

What confirmation still requires

A dwarf planet must be massive enough for its own gravity to pull it into a round shape, and size is the open question for this Rubin dwarf planet candidate. Based on its distance, brightness and estimated surface reflectivity, the team estimates the object spans between 600 and 1500 kilometres, according to the observatory. Icy bodies that large are expected to be round, which is why the team treats the object as a strong candidate. Infrared observations are still needed to narrow the size range and settle its status.

If confirmation comes, the candidate would join five worlds recognized by the International Astronomical Union: Pluto, Ceres, Eris, Haumea and Makemake. That small roster shows why a single addition matters to planetary scientists. Each confirmed dwarf planet adds a data point about how the outer solar system formed, how giant planets moved over time and how smaller bodies were scattered during that evolution.

The discovery also shows what Rubin can do as its main survey gets under way. The Legacy Survey of Space and Time began in June, and earlier in the year the Rubin solar system team submitted observations of more than eleven thousand new asteroids to the Minor Planet Center, including roughly three hundred eighty new trans-Neptunian objects. Team members called that submission a hint of what was coming. With this candidate, reported by the NSF and the observatory, that hint has turned into a named object that other telescopes can now study.

Scientists around the world can now organize follow-up observations to learn more about the Rubin dwarf planet candidate, including its composition and exact size. The public can explore its orbit through Orbitviewer, an interactive web-based app that shows objects in the solar system in three dimensions. Rubin will image the whole southern sky again and again over the coming decade, so other distant bodies hiding in places earlier surveys skipped may soon come into view as well. Sources for this article include the observatory announcement at Rubin Observatory and the NSF announcement at NSF.