Amateur astronomer Sam Deen says he found 2026 ED138 in two 1954 images

Amateur astronomer Sam Deen has traced dwarf-planet candidate 2026 ED138 to 2 photographic images taken at California’s Palomar Observatory in 1954, Science News said on 30 October. Other amateur astronomers independently found the same sightings.

The previously known record began with Dark Energy Camera images from 2016.

From survey images to discovery

The Vera C. Rubin Observatory observed the object 30 times on 20 nights between 4 March and 11 July 2026. Matt Holman, an astrophysicist at the Smithsonian Astronomical Observatory and a Rubin Operations contributor, identified it on 13 October 2026 using software that matches moving points of light across images. The Minor Planet Center announced the discovery on 27 October 2026.

Rubin places 2026 ED138 about 46 astronomical units from the Sun. An astronomical unit is the average distance between Earth and the Sun. Science News said the object’s slow apparent motion and its position against a crowded background of Milky Way stars helped it escape detection; it takes approximately 280 years to orbit the Sun.

Why older images help

Finding an object in images taken before its discovery is called precovery. For 2026 ED138, the historical sightings supply positions far apart in time. An orbit must fit those earlier positions as well as the recent ones, narrowing the range of possible paths and improving predictions of where the object will appear next.

Its size remains an estimate

A better orbit does not directly measure the object’s diameter. Rubin’s discovery team estimates a diameter of 600–1,500 kilometers from its distance, brightness and assumed surface reflectivity, called albedo. For the same measured brightness, a more reflective surface implies a smaller object; a darker surface implies a larger one.

A dwarf planet orbits the Sun directly, has enough mass for its own gravity to make it round, and does not gravitationally dominate its orbital neighborhood. Rubin’s team expects icy bodies in the estimated size range to be round, but 2026 ED138’s size and shape remain unconfirmed.

The Halloween observation

Deen hopes to observe 2026 ED138 pass in front of a distant star on 31 October 2026. Such a passage, called a stellar occultation, could reveal details of the object’s size and shape, or evidence of moons or rings, if observers obtain usable measurements.

Deen said astronomers usually need years to determine a newly discovered object’s orbit well enough to predict an occultation. The long archival record makes this early opportunity unusual. Beyond that attempt, Rubin’s team says observations in infrared light are needed to constrain the object’s exact size and confirm its nature as a dwarf planet.