Rubin Observatory Spots Possible New Dwarf Planet Beyond Pluto

Astronomers using a telescope in Chile have found an icy object beyond Pluto that may join the short list of dwarf planets in our solar system. The candidate, named 2026 ED138, still needs more study before its status can be confirmed.

Researchers estimate the object's diameter at somewhere between roughly 600 and 1,500 kilometres. It orbits about 46 times farther from the sun than Earth does. Pluto, by comparison, circles the sun at an average distance of about 40 times that of Earth.

The discovery was made with the new Vera C. Rubin Observatory, which sits on the summit of Cerro Pachón in central Chile. The telescope is a joint initiative of the U.S. National Science Foundation and the U.S. Department of Energy. The finding was announced at a scientific meeting in Spokane, Wash.

If confirmed, 2026 ED138 would become the sixth dwarf planet recognized by the International Astronomical Union, the global authority on classifying celestial bodies. The current five are Pluto, Eris, Haumea and Makemake, all of which orbit beyond Neptune, and Ceres, which lies in the main asteroid belt between Mars and Jupiter. Based on present data, the new candidate would rank among the largest of them.

Matt Holman, an astrophysicist at the Center for Astrophysics, Harvard & Smithsonian, was the first to spot the object. "For me, what this demonstrates is that many exciting discoveries remain to be made with the Rubin Observatory and other telescopes," he said.

Pluto was long regarded as the solar system's ninth and outermost planet. The International Astronomical Union reclassified it as a dwarf planet in 2006 because it did not satisfy the organization's formal definition of a planet. Pluto and the new candidate both lie in a frigid region of the solar system populated by many icy bodies.

Questions remain about the size and makeup of 2026 ED138. To qualify as a dwarf planet, an object must have enough mass for its own gravity to pull it into a round shape. The candidate appears to meet that standard, but more observations are required to be sure.

"The main thing we need to do is to establish the physical size of 2026 ED138," Holman said. He explained that Rubin observes sunlight reflected from the object, which creates ambiguity. "A smaller, more reflective object would show the same amount of light as a larger, less reflective object."

The solution, Holman said, is to look at longer wavelengths with an observatory such as ALMA, the Atacama Large Millimeter/submillimeter Array, another powerful telescope based in Chile. Such an instrument can detect light emitted by the object itself, which helps determine its size.