Rubin Observatory in Chile Finds Possible New Dwarf Planet

Astronomers at the Vera C. Rubin Observatory in Chile have reported a new object in the solar system that may turn out to be a dwarf planet. It has been given the provisional designation 2026 ED138. If it meets the criteria, it would be added to the list of officially recognized dwarf planets.

The object is estimated to measure between 600 and 1,500 kilometers across. It lies about 46 astronomical units from the sun, where one astronomical unit is roughly the distance between Earth and the sun. By comparison, Pluto orbits at an average of 40 astronomical units. ED138 belongs to the trans-Neptunian objects, which lie beyond Neptune's orbit, and like Pluto its orbit is tilted relative to the plane in which the planets travel. Its discoverers say it could be among the brightest and largest objects of its kind.

How it was found

Matthew Holman, a researcher at the Harvard-Smithsonian Center for Astrophysics and a member of the observatory's operations team, spotted the object on October 13 while analyzing data Rubin gathered in 2026. A deeper search turned up additional observations in archives going back to 2016, allowing the team to reconstruct more than a decade of its trajectory.

Astronomers explain that ED138 stayed hidden because it sits in the sky against the disk of the Milky Way, a thick band of millions of stars. That concentrated glow makes fainter objects hard to pick out, but the huge volume of Rubin data made it possible to identify it.

What remains to be shown

So far, only its status as a trans-Neptunian object is established. Researchers need a more precise size to determine whether its own gravity is strong enough to give it a nearly spherical shape. Unlike a planet, a dwarf planet has not cleared its orbit of other bodies of comparable size, which is why Pluto lost its planet status in 2006. The International Astronomical Union currently recognizes five dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris.

ED138 is not the only candidate. Gonggong, Quaoar, Sedna and Orcus are among the best known, and experts acknowledge the list could be longer. Confirmation is difficult because of the distances involved: to obtain detailed images of Pluto, the New Horizons probe travelled nearly nine and a half years, from its 2006 launch to its 2015 flyby.