A telescope can photograph an object without anyone knowing it is there. It takes more than a bright dot in one image to recognize a distant world: astronomers have to connect observations made on different nights and show that the dots follow a consistent path.
That is how 2026 ED138 emerged from images taken by the Vera C. Rubin Observatory. The object exists, and its orbit is increasingly well constrained. The open question is whether it is large enough to be a dwarf planet. Its measured brightness cannot settle that on its own.
Discovery came after the photographs
Rubin recorded 2026 ED138 between March 4 and July 11, 2026. On October 13, astrophysicist Matthew Holman identified it while running a program, developed with Kevin Napier, that looks for moving objects across observations. The observatory announced the find on October 27.
The months between photography and recognition matter. A single point of light could be confused with a star, an image artifact, or an unrelated object. A sequence that moves as a Solar System body should gives researchers something they can test. Holman and colleagues located 30 Rubin observations from 20 nights, then found the object in older images taken with the Dark Energy Camera. According to their research preprint, those archival detections extend the observed track back to 2016.
Holman told Smithsonian magazine that the team also found the object on photographic plates from 1937 in Harvard’s collection. The object was recorded then, but recognized only after researchers knew where to look.
An orbit tells us where it goes, not how big it is
The object was about 46 astronomical units from the Sun when Rubin announced it. One astronomical unit is roughly the Earth–Sun distance. That 46-au figure describes its location at the time, while the paper’s 43.09-au figure is the semimajor axis, a measure of its orbit’s overall size. Its path tilts about 24 degrees relative to the main plane of the Solar System.
Finding it in images across many years helps astronomers test and refine that path. It does not provide a tape-measure reading of the body itself. At such a distance, 2026 ED138 appears as a point of light. Its brightness depends both on its size and on how much sunlight its surface reflects.
The discovery team calculates a diameter between 612 and 1,530 kilometers (about 380 to 950 miles) by assuming different plausible reflectivities. A smaller, reflective body and a larger, darker one could send similar amounts of sunlight toward Earth. That is why the range is an estimate built from assumptions, not a measured diameter.
How could astronomers measure its size?
Under the International Astronomical Union’s definition, a dwarf planet orbits the Sun, has enough gravity to become nearly round, has not cleared its orbital neighborhood, and is not a moon. Researchers expect an icy body in the estimated size range to be round. They still call 2026 ED138 a candidate because its actual size and shape have not been established.
Two observations could narrow the size question. A thermal measurement would detect heat the object emits, providing information that reflected sunlight alone cannot. An occultation would catch it passing in front of a background star: the duration and geometry of the star’s disappearance could constrain the width of the object’s silhouette. As Smithsonian reports, astronomers are pursuing observations of this kind. For now, its size remains an estimate, and a better measurement would help test whether it is large enough to have become nearly round.
Rubin’s find therefore has two distinct results. The moving dot has become a well-supported object with an orbit, and years of overlooked images are helping trace its route. A better orbit tells observers where to aim. A size measurement will help tell them what they have found.