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In numbers
46au
Distance of 2026 ED138 from the Sun in astronomical units
30observations
Number of Rubin observations of the object between March and July 2026
25°
Inclination of the orbit of the object to the plane of the Solar System in degrees
The Rubin Observatory has found an object in this year’s data, 2026 ED138, a candidate for the sixth dwarf planet. It has been captured in DECam archive images since 2016. But it lies close to the plane of the Milky Way, and among the multitude of background stars it escaped most telescopes.
01Announcement
What Rubin has announced and what it has not
Astronomer Matthew Holman came across the object on the evening of 13 October 2026, while going through data that Rubin had collected earlier this year. The Minor Planet Center of the International Astronomical Union then announced the observations on 27 October. In all, Rubin caught it 30 times over 20 nights between 4 March and 11 July 2026.
The object is now about 46 astronomical units from the Sun, that is 46 times farther than the Earth, whereas Pluto orbits at an average distance of 40 units.
Not every trans-Neptunian object is a dwarf planet candidate, because size and shape decide. A dwarf planet has to be massive enough for its own gravity to pull it into a round shape.
The distance of the object has been measured, but its diameter is only estimated, at 600 to 1500 km. If it is confirmed as a dwarf planet, ED138 would be among the largest dwarf planets discovered in this region of the Solar System.
02Interesting facts
An object that spent years hiding among the stars
In that crowded star field Holman found it with code he developed with his colleague Kevin Napier. The code works through billions of detections of moving objects, compares them and looks for those that belong to the same body.
Mario Jurić then tracked down earlier observations in Rubin's catalogues and images reaching back to 4 March 2026, 20 of the 30 in total. DECam archive images supplied data going back to 2016, letting the team follow the object's orbit back over more than ten years. But the discovery rests on this year's Rubin data, and the archive only confirmed it after the fact.
The orbit of ED138 is inclined by almost 25 degrees to the plane of the Solar System. That is why astronomers place it among the so-called hot classical trans-Neptunian objects, bodies beyond Neptune with a markedly tilted orbit. The inclination also suggests that the gravity of Neptune pushed it out of its original orbit.
03What next
Is it round? The hope for a sixth rests on that
A range of 600 to 1500 km is a large uncertainty, because the size of such a distant object cannot be measured directly. The estimate is based on its distance, its brightness and the estimated reflectivity of its surface, known as the albedo. At the same brightness, a dark object has to be larger than a light one.
To refine the size and to confirm that it is a dwarf planet, observations in the infrared are needed. For such large icy bodies a round shape is expected, but an expectation is not yet a measurement.
The International Astronomical Union currently recognises five dwarf planets, namely Pluto, Ceres, Eris, Haumea and Makemake. It moved Pluto into this group in 2006, and in the same year Ceres was also classified as a dwarf planet. ED138 would join them if its nature is confirmed.
04Summary
What is certain and what is not yet
What is certain is that the object exists and lies about 46 au away. Whether it is round, and so a dwarf planet, will be decided by further measurements. It is too faint for an ordinary amateur telescope.