Rubin finds a dwarf planet candidate in a crowded star field

The newly reported object, 2026 ED138, is real. Whether it qualifies as a dwarf planet depends on physical properties that still need measuring.

A newly reported object beyond Neptune could be a dwarf planet, after astronomers found it in Vera C. Rubin Observatory images of a densely populated star field. The object, designated 2026 ED138, had escaped earlier searches in a region crowded with background stars.

Rubin announced the discovery on October 27. A detailed research preprint posted October 28 describes checks of the detection and estimates of its physical properties. The paper has been submitted to The Astronomical Journal; it is not presented here as a peer-reviewed result.

A moving dot among background stars

Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian identified ED138 while searching previously collected Rubin data on October 13. Software linked detections made on different nights into the track of a single moving object.

The team ultimately identified 30 Rubin observations on 20 nights between March 4 and July 11, 2026. Archival images from the Dark Energy Camera supplied additional measurements from 2016 onward. Those older records helped test and extend the orbit.

Rubin’s announcement places the object about 46 astronomical units from the Sun. One astronomical unit is approximately the Earth–Sun distance. The discovery shows how repeated imaging and searches through older observations can expose objects hidden in crowded fields.

Why its size remains uncertain

The team’s estimated diameter is roughly 600–1,500 kilometers for one assumed range of surface reflectivity, or albedo. That is not a direct measurement of its width: a small, reflective object can appear as bright as a larger, darker one.

The preprint considers a size nearer 600–970 kilometers more probable, but also allows a smaller object if its surface is unusually reflective. Its composition and shape remain unresolved, so the dwarf planet label stays provisional.

Thermal observations or a stellar occultation, when the object passes in front of a background star, could measure its size more directly. Until then, the discovery establishes a new object for follow-up, rather than a confirmed addition to the recognized dwarf planets.

Sources: Rubin Observatory announcement and Holman and colleagues’ preprint, version 1. Reporting checked October 30, 2026, Los Angeles time.