At first glance, 2026 ED138 is a point of light among a crowd of stars. Its significance becomes apparent when images taken on different dates are compared: the point moves. Astronomers at the Vera C. Rubin Observatory have thus identified a distant object in the Solar System, announced on 27 October 2026, which could join the family of dwarf planets. But the discovery leaves two quite distinct questions: where is this object, and how large is it really?
The first question already benefits from older images that make it possible to trace its trajectory back in time. The second remains open. There is still work to do between a convincing detection and a physical description of a world — and that is precisely what makes this discovery interesting.
A recent discovery in old images
On 13 October, Matthew Holman, of the Center for Astrophysics | Harvard & Smithsonian, examines Rubin data using a programme developed with Kevin Napier. He is looking to link scattered detections: could several points recorded at different times belong to the same moving object? One combination catches his attention. Its distance and brightness suggest an unusually large body.
Mario Jurić then finds other observations in the observatory’s archives. In total, Rubin’s Simonyi Survey Telescope recorded the object thirty times, over twenty nights between 4 March and 11 July 2026. Images from the DECam camera on the Blanco Telescope in Chile make it possible to trace it back to 2016. The announcement is new; some of the light that made it possible had therefore been recorded ten years ago. The observatory details this timeline.
Why had an object this bright escaped previous searches?
The word “bright” needs a little clarification here. It does not mean that 2026 ED138 is easily visible to the naked eye. In their study submitted on 28 October, Holman and his colleagues rank it among the ten intrinsically brightest known trans-Neptunian objects: a comparison that takes distance into account. “Trans-Neptunian” describes an object whose orbit lies beyond Neptune.
The sky has not been surveyed everywhere to the same depth or at the same time. The authors show that 2026 ED138 lies far away in the southern sky, in a direction crowded with stars from the Milky Way. Some earlier searches did not reach this area; others avoided the densest fields. A region may have been photographed without a moving object in it being recognised.
The difficulty is like looking for a face in several photographs of crowds: you have to isolate it, then establish that it is indeed the same individual from one image to the next. For astronomers, movement provides an essential clue. Rubin’s contribution therefore lies in the processing of observations as well as in the power of a telescope. The limitations of previous searches are described in the discovery study, particularly section IX.
Between 600 and 1 500 kilometres: an estimate to understand
The announced range invites us to imagine a globe straight away. Yet the researchers have not measured a disc 600 to 1 500 kilometres in diameter in a detailed photograph. They start from the object’s brightness and make assumptions about the amount of light reflected by its surface, known as albedo.
At a comparable distance, a relatively small but highly reflective body can return as much light as a larger, darker one. Without knowing its reflectivity, its brightness cannot therefore be converted into a single diameter. The study calculates about 612 kilometres for an albedo of 0,25, 968 kilometres for 0,10 and 1 530 kilometres for 0,04. These are three assumptions about the surface applied to the same brightness measurement, not three conflicting measurements of its size.
The authors consider a size between 612 and 968 kilometres more likely, while retaining broader possibilities. They also point out that the surface composition remains unknown. Focusing only on the highest value would erase the uncertainty that gives the result its meaning. These distinctions appear in the section on size and albedo.
“Candidate” is not just a matter of wording
NASA notes that a dwarf planet orbits the Sun and has an approximately round shape, but has not cleared its orbital neighbourhood in the way planets have. Ceres, Pluto, Haumea, Makemake and Eris are the five objects currently recognised in this category by the International Astronomical Union. The distinction is explained in its guide to dwarf planets.
For 2026 ED138, the dimensions under consideration make a shape moulded by its own gravity plausible. They do not prove it. The study emphasises that information about its shape or variations in its brightness as it rotates, in particular, would be needed to characterise it better. An object that has been detected with certainty can therefore retain a provisional classification: these are two different levels of knowledge.
Nor is this an announcement that would increase the number of major planets in the Solar System to nine. The point is to understand an additional distant body, its properties and history, without prematurely assigning it a definitive place in the classification.
TESS also finds the same object
A second study, submitted on 29 October by Te Han, Jeroen Audenaert and their colleagues, draws on the archives of the TESS satellite. It finds 2026 ED138 in five observation periods between 2019 and 2026, four of them before the discovery. This is not simply a matter of spotting an obvious point in an image: the object is too faint to be located in a single TESS exposure.
The team removes the contribution from known stars, then adds images together after shifting them according to the expected trajectory. This can strengthen the signal from the moving body. The positions obtained with TESS support the orbit calculated from the other observations. This new series of data provides a useful check, without resolving the question of diameter on its own. The method and its tests are described in the TESS study. These two studies are scientific preprints; they should be read with their methodological limitations in mind.
What future observations could change
To distinguish between the possible sizes, the researchers mention measurements of thermal radiation or a stellar occultation: the object passing in front of a star, whose temporary disappearance can provide information about its dimensions. Studying light at different wavelengths could also shed light on the nature of its surface. These are possible avenues of measurement, not results already obtained or a guaranteed observation schedule.
The significance of 2026 ED138 thus goes beyond its possible classification. It shows that a region already observed can still yield major objects when new methods link images together. The next advances will be reflected in very concrete data: a more tightly constrained size, a rotation curve, clues to its composition. These are the details that will mark the transition from a well-identified point of light to a better-understood world.
Photograph: the Vera C. Rubin Observatory in Chile. NSF–DOE Vera C. Rubin Observatory/NOIRLab/SLAC/AURA/H. Stockebrand — Wikimedia Commons, CC BY 4.0.
