2026ED₁₃₈

2026 ED138 is a trans-Neptunian object: a body of ice and rock orbiting the Sun out past Neptune, in the same region as Pluto. It was announced on 27 October 2026 in Minor Planet Electronic Circular 2026-U85 and a joint Rubin Observatory and NOIRLab press release, and it is the first dwarf planet candidate to come out of the Vera C. Rubin Observatory in Chile.

It is officially a candidate. To count as a dwarf planet it has to be massive enough to have pulled itself round, and nobody has measured its size yet. If it passes, it would join the five dwarf planets the International Astronomical Union recognises today: Ceres, Pluto, Haumea, Makemake and Eris (Rubin).

The name is a provisional designation, not a name: "2026" is the year, and "ED138" encodes when in the year it was first recorded and its place in that batch. A real name comes later, after the Minor Planet Center gives it a permanent number (IAU naming rules).

Matthew Holman of the Harvard and Smithsonian Center for Astrophysics spotted it on the evening of 13 October 2026, using linking software called pointdexter that he wrote with Kevin Napier. It joins up faint detections from different nights that belong to one slow, distant object, and it runs inside Rubin’s Solar System pipelines (Holman et al. 2026). The first link was 10 detections on 8 nights in May and June 2026. Mario Jurić at the University of Washington then found 20 more in Rubin data going back to 4 March, for 30 detections on 20 nights (Rubin). Jurić called it “something that happens maybe once in a generation”.

Why did nobody find something this bright before? It sits far south in the sky (around declination −42°, in Corona Australis) and only about 12° from the plane of the Milky Way, where crowded star fields swamp faint moving dots. In Rubin’s own data, most visits covering it had no clean catalogue entry because it blended with stars (Holman et al.). The same search turned up 2,440 previously unknown trans-Neptunian objects; ED138 is the brightest of them.

Once its orbit was known, astronomers and amateurs went back through old images. It has now been found on photographic plates from 1937 (Boyden Observatory, South Africa), 1954 (Palomar) and later Siding Spring surveys, in VISTA infrared images from 2010 and 2011, in DECam images from 2016 to 2023, and in five sectors of NASA’s TESS mission (JPL, Rubin Community, Han et al. 2026). That gives an observation arc of about 88 years, the longest of any trans-Neptunian object except Pluto (Sky & Telescope).

We do not know yet, and anyone giving one exact number is guessing. Astronomers only have its brightness, and brightness depends on both size and how reflective the surface is. Working from its brightness, the discovery paper gives about 612 km if the surface reflects 25% of sunlight, 968 km at 10%, and 1,530 km at 4%, and calls 612 to 968 km the more likely range (Holman et al.). Press releases round this to 600 to 1,500 km.

Even the low end is above the roughly 450 km at which an icy body is expected to pull itself round, which is why it is a serious dwarf planet candidate. One caveat from the paper: a very bright, methane-frosted surface would shrink it to about 320 to 400 km. Its colour is red, which Jurić reads as ice darkened by organic material (National Geographic).