2026 ED138: A new candidate Dwarf Planet in the Kuiper Belt.

In the late twentieth century, our model of the Solar System was relatively simple, with nine planets orbiting the Sun, some of which had moons and rings, plus a variety of smaller comets and asteroids. The smallest of these planets, Pluto, was somewhat of an oddity, being smaller several of the Solar System's moons, including our own, and having a highly eccentric orbit which crossed the orbit of another planet, Neptune, but was clearly still to large a be a comet or asteroid. However, in the early twenty first century, this model began to change, with the discovery of a series of Pluto-like bodies beyond Neptune starting with Quaoar in 2002, then Sedna in 2003, Orcus and Haumea in 2004, and Eris and Makemake in 2005. This led the International Astronomical Union to reconsider how bodies within the Solar System are classified, coming up with a new category of objects, 'dwarf planets', in 2006. 

Dwarf planets are defined as objects large enough to have become rounded under the influence of their own gravity, but not large enough to have cleared their orbital path of other objects. Pluto was reclassified as a dwarf planet, losing its status as the Solar System's smallest planet, as was Ceres, the largest body in the Main Asteroid Belt, along with the newly discovered bodies Quaoar, Sedna, Orcus, Haumea, Eris, and Makemake. This list has subsequently grown with the addition of bodies such as Salacia and Gonggong.

In a press release issued on 27 October 2026, the Vera C. Ruben Observatory, which is located on Cerro Pachón in Chile, announced the discovery of a new candidate dwarf planet, 2026 ED138, which was discovered by astronomer Matthew Holman of Harvard University, who was analysing data collected by the Ruben Observatory, looking for moving objects in the outer Solar System.

Holman found 30 observations of the new object made by the Ruben Observatory between 4 March and 11 July 2026, which provided him with enough data to discover additional sightings in imagery from the Dark Energy Camera on the the Víctor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory, and the Asteroid Institute’s ADAM Precovery service, with the oldest images dating back to 1937. 

A mosaic of images captured by NSF–DOE Vera C. Rubin Observatory of the newly discovered dwarf planet candidate 2026 ED138. 2026 ED138 was discovered in observational data from Rubin spanning 4 March–11 July 2026. It was found by Rubin Operations team member and in-Kind contributor Matthew Holman, who contributes to improving Rubin's algorithms that search for distant moving objects in our Solar System. Its distance from the Sun places it beyond the orbit of Pluto, and initial analysis indicates that it may be among the largest and brightest bodies scientists have discovered in this region of the Solar System. Vera C. Rubin Observatory/NOIRLab/SLAC National Accelerator Laboratory/AURA Image Processing/Mario Jurić/Davide de Martin/Mahdi Zamani.

The designation 2026 ED138 implies that it was the 2454th asteroid (object D138 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended so that A = 1, A1 = 26, A2 = 51, etc., which means that D138 implies the (138 x 25 + 4) 2454th asteroid) discovered in the first half of March 2026 (period 2026 E - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

2026 ED138 is currently about 47.225 AU from the Sun (where 1 AU is the average distance between the Earth and the Sun), with an orbital period estimated at 281 years, which will bring it as close as 36.86 AU from the Sun and as far as 49.04 AU. As an object which has an orbit entirely outside the orbit of Neptune, it is a trans-Neptunian Object, and having a low-eccentricity (approximately circular) orbit, it is a Classical trans-Neptunian Object. This orbit is tilted at 24.5° to the plain of the Solar System, making 2026 ED138 a Hot Classical trans-Neptunian Object; a Classical trans-Neptunian Object with an orbit tilted at more than 5° to the plain of the Solar System. Such objects are thought to have been disturbed from the plain of the Solar System by gravitational interactions with the planet Neptune.

The relative positions of 2026 ED138, the Earth, and the planets of the Outer Solar System on Sunday 1 November 2026. JPL Small Body Database.

2026 ED138 is a bright object, with an absolute magnitude estimated to be between 2.85 and 3.19. This would make it the ninth or tenth brightest trans-Neptunian Object discovered to date. From this brightness it is estimated that it has a diameter of between 600 and 1500 km. This variation stems from an uncertainty as to whether it has a bright surface, which would place it at the lower end of this range, or a dull one, which would place it towards the upper end, and make it a Dwarf Planet. Potential observations of the object with infrared telescopes, observations of stellar occulations (events in which it passes in front of a star), and possibly direct observations with the James Webb Space Telescope could tell us more about its nature.

The orbital period of 2026 ED138 suggests that it is locked into a 12⁄7 orbital resonance with Neptune. This is roughly the same as that of the dwarf planet Haumea, as well as a group of objects called the Haumean Family of trans-Neptunian Objects. However, it is not currently thought to be a member of that family due to the difference in its albedo (brightness) to that of Haumea and the Haumean Family objects.

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