Rubin finds dwarf-planet candidate 2026 ED138 beyond Pluto; its size is still an estimate

The 2026 ED138 announcement rests on 30 Rubin observations and 44 archival DECam detections spanning ten years, but its quoted 612 to 1,530 km diameter is calculated from brightness and an assumed reflectivity, not measured.

Official announcement · Preprint · Dwarf-planet candidate, not confirmed

A large, bright object orbiting beyond Pluto, provisionally named 2026 ED138, is the first dwarf-planet candidate from the NSF–DOE Vera C. Rubin Observatory. Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian found it on 13 October in Rubin data; the IAU Minor Planet Center published the discovery on 27 October and a 22-author preprint followed a day later. If its size holds up, it would join the five dwarf planets the IAU recognises. The catch: nobody has measured that size. The quoted diameter comes from brightness and an assumption about how dark its surface is.

Evidence at a glance

Is this a big deal?

A solid find, and a rare one. Mario Jurić of the University of Washington, who leads Rubin’s Solar System team, told UW News that finding something roughly 1,000 km across “is something that happens maybe once in a generation.” The dwarf-planet label, though, is a prediction from size, and the size is a prediction from brightness. Until infrared or occultation data arrive, “possible sixth dwarf planet” is a well-grounded candidacy, not a classification.

What scientists found

2026 ED138 is a trans-Neptunian object, a body that orbits the Sun beyond Neptune. The MPC’s orbit puts it on a 281-year path tilted 24.5 degrees to the plane of the planets, between 36.9 and 49.0 au from the Sun; it last passed closest to the Sun in February 1926. NOIRLab gives its current distance as about 46 au. The MPC’s ephemeris for 27 October says 47.2 au, and SciQuest’s recalculation from the MPC orbit agrees. Light reflected from it takes about 6.5 hours to reach Earth.

What makes it stand out is brightness. The preprint measures an absolute magnitude, the brightness an object would have at a standard distance, of H_V = 3.19 ± 0.04. Converted to a diameter, that gives 612 km if the surface reflects 25% of sunlight, 968 km at 10%, and 1,530 km at 4%.

How they found it

On the evening of 13 October, Holman was running Rubin data from earlier in the year through a code he wrote with his CfA colleague Kevin Napier, named pointdexter in the preprint. It groups detections of moving points of light that belong to one object. ED138 first appeared as ten detections on eight nights in May and early June. “My first reaction was one of excited disbelief,” Holman said in NOIRLab’s release. “Objects like this are very rare, so I immediately started checking whether I or the code had done something wrong.”

Jurić then searched Rubin’s catalogues and images and found 20 more detections, the earliest from 4 March. The Asteroid Institute’s ADAM precovery service pointed to older images from the Dark Energy Camera on the Blanco 4-metre telescope at Cerro Tololo: 44 detections between February 2016 and April 2023. That arc, 3,789 days, is why the orbit is already secure.

Why did nobody see it earlier? It sits far south, at a declination near minus 43 degrees, about 12 degrees from the plane of the Milky Way, where crowded star fields swamp faint movers. Even Rubin’s automated catalogues missed it in 24 of 38 visits because its light blended with neighbouring stars, the preprint says. At about magnitude 19.9 on the MPC’s ephemeris, it is roughly 350,000 times fainter than the faintest naked-eye star.

Why it matters to you

Nothing in daily life changes. The payoff is a better inventory of our own neighbourhood: how many sizeable worlds the Sun holds, and where they are. Rubin’s ten-year survey will test the preprint’s estimate of how many more are out there. For a sense of how far 47 au is, see how long it takes to reach each planet; New Horizons needed nine and a half years to reach Pluto.

What the evidence does not show

It does not show that 2026 ED138 is round. The IAU’s 2006 definition requires a body to have enough mass for its own gravity to pull it into a “hydrostatic equilibrium (nearly round) shape.” The preprint notes that even 612 km exceeds the roughly 450 km threshold at which icy bodies are expected to relax into that shape, but adds that a light curve or a shape measurement is still needed.

Think of a lamp seen across a valley at night. You cannot tell a small bright bulb from a large dim one. ED138’s brightness cannot separate a 612 km body reflecting a quarter of its sunlight from a 1,530 km body as dark as charcoal. The analogy stops where thermal infrared starts: a body’s heat glow depends on its size, so infrared or occultation measurements break the tie. A surface as reflective as fresh ice (albedo 0.6 to 0.9) would shrink it to about 320 to 400 km, a case the preprint calls less likely.

How this fits what we already knew

The IAU’s list has five names: Pluto, Ceres, Eris, Haumea and Makemake. NASA’s fact pages give Pluto 2,377 km across and Eris about 2,400 km, so even the dark end of ED138’s range would leave it well behind those two and closer to Makemake at about 1,430 km. Other large TNOs such as Quaoar and Gonggong are not on the list either, so “sixth dwarf planet” means sixth recognised by the IAU.

The orbit adds a puzzle. The preprint finds it at the edge of the 12:7 mean-motion resonance with Neptune, the same neighbourhood as Haumea, though only about 46% of simulated clones stay resonant over 10 million years. Its colour, g − r = 0.63, is redder than the Sun’s 0.41, which the authors say rules it out as a fragment of Haumea’s collisional family. Rubin’s earlier haul in this region includes Neptune’s L5 Trojans.

Independent perspective

SciQuest has not yet interviewed an astronomer outside the discovery team. Michele Bannister of the University of Canterbury in New Zealand, who is not among the preprint’s authors, told CNN that the object’s size is just within the range where it can be thought of as a world, and that for now all anyone knows is that it is big; the rest, she said, will be speculation for a while.

How other headlines framed it

• Scientific American led with a possible sixth dwarf planet and kept the candidate status throughout, quoting only Holman.
• Gizmodo put “the biggest in 20 years” in its headline; the preprint ranks ED138 among the ten intrinsically brightest TNOs and says the exact rank depends on whose magnitudes are compared, with the MPC’s H of 2.8 differing from the team’s 3.19.
• EarthSky framed the story as a question, added the Neptune resonance, and cited an orbit-simulation hobbyist for it.
• IFLScience stressed the need for infrared follow-up and compared the 25-degree tilt with Pluto’s 17.
• CNN, via Scripps, called it a “startling discovery” but was the only outlet to carry an outside view, from Bannister.

Most outlets give the distance as 46 au, following the release; the MPC’s ephemeris puts it at 47.2 au.

What happens next

The preprint lays out the follow-up: a near-infrared spectrum to look for water ice and other volatiles, light curves for rotation and shape, thermal measurements for size and albedo, stellar occultations, and a search for a moon. It is bright enough for Hubble and JWST, the authors note. Holman told CNN that pinning down its physical size will probably take a few months. Rubin, meanwhile, will image the southern sky repeatedly for ten years.

In one sentence

Rubin has found a rare, intrinsically bright body beyond Pluto whose orbit is secure, but its 612 to 1,530 km size and its dwarf-planet status hinge on a reflectivity nobody has measured yet.

Read next

• Rubin’s Neptune Trojan discoveries: the same survey, one step closer to the Sun.
• How long does it take to get to each planet? Travel times that put 47 au in perspective.
• Gravity on other planets: why mass, not distance, decides whether a body pulls itself round.

Sources and disclosure

1. IAU Minor Planet Center. “M.P.E.C. 2026-U85: 2026 ED138.” Issued 27 October 2026, 18:17 UT. Orbit record: MPC database entry for 2026 ED138.
2. Holman M. J., Jurić M., Napier K. J. and 19 co-authors. “2026 ED138: A Large Trans-Neptunian Object in Early Rubin Observatory Data, and How Many More May Remain.” arXiv:2610.38693v1, submitted 28 October 2026; the comments field says submitted to The Astronomical Journal. Not peer reviewed.
3. NSF NOIRLab. “NSF–DOE Vera C. Rubin Observatory Reveals Its First Dwarf Planet Candidate in Our Solar System.” Science release noirlab2626, 27 October 2026 (source of the Holman quotes).
4. University of Washington News. “Rubin Observatory reveals its first dwarf planet candidate in our solar system.” 27 October 2026 (source of the Jurić quote). Also carried by SLAC National Accelerator Laboratory.
5. IAU. “Resolution 5A: Definition of a Planet in the Solar System,” 2006, as archived by NASA’s National Space Science Data Center.
6. NASA. Planetary Fact Sheet (Earth, Neptune and Pluto distances); fact pages for Pluto, Eris, Haumea, Makemake and Ceres; and the New Horizons mission page (launch 19 January 2006, Pluto flyby 14 July 2015).
7. Prisco J. “Astronomers make startling discovery of a possible dwarf planet beyond Pluto.” CNN, via Scripps News (source of the Bannister and Holman remarks paraphrased above).

Disclosure: This article was drafted with AI assistance and checked against the sources above, including the full preprint. Rubin Observatory is funded by the US National Science Foundation and the Department of Energy’s Office of Science. Calculations by SciQuest: heliocentric distance on 27 October 2026 from the MPC elements (a = 42.949 au, e = 0.1417, mean anomaly 128.33° at epoch 9 June 2026) gives 47.22 au, matching the MPC ephemeris; 47.22 au × 149.6 million km = 7.06 billion km, and × 499 light-seconds = 6.5 hours; diameters from D = 1,329 km × 10^−H/5 ÷ √albedo reproduce the preprint’s 612, 968 and 1,530 km; the arc from 25 February 2016 to 11 July 2026 is 3,789 days; magnitude 19.9 against a naked-eye limit of 6 is 10^13.9/2.5 ≈ 360,000 times fainter, rounded to 350,000; Ceres’s diameter is twice NASA’s 476 km radius; New Horizons’ 19 January 2006 launch to 14 July 2015 flyby is 9.5 years. The graphic is a SciQuest original drawn from the cited values. Spotted an error? See our corrections policy.