2026 ED138: A Large Trans-Neptunian Object in Early Rubin Observatory Data, and How Many More May Remain

Matthew J. Holman; Mario Jurić; Kevin J. Napier; Jacob A. Kurlander; Aren Heinze; Joachim Moeyens; Megan E. Schwamb; Pedro H. Bernardinelli; Ian Sullivan; Eric C. Bellm; Silvia Protopapa; Colin Orion Chandler; Ian Chow; Eric J. Christensen; Samuel Cornwall; Siegfried Eggl; Sarah Greenstreet; Chester Li; Erfan Nourbakhsh; James E. Robinson; Devanshi Singh; Danica Zilkova

AI summary · arXiv: 2610.38693v1 · astro-ph.EP

2026 ED138: A Large Trans-Neptunian Object in Early Rubin Observatory Data, and How Many More May Remain

Matthew J. Holman, Mario Jurić, Kevin J. Napier, Jacob A. Kurlander, Aren Heinze, Joachim Moeyens, Megan E. Schwamb, Pedro H. Bernardinelli,

and 14 more authors

Ian Sullivan, Eric C. Bellm, Silvia Protopapa, Colin Orion Chandler, Ian Chow, Eric J. Christensen, Samuel Cornwall, Siegfried Eggl, Sarah Greenstreet, Chester Li, Erfan Nourbakhsh, James E. Robinson, Devanshi Singh, Danica Zilkova

Original paper Submitted to arXiv on 28 Oct. 2026 30 pages, 9 figures, 12 tables. Submitted to AJ License: CC BY 4.0 Also in: fr

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The essentials

Early Rubin data revealed 2026 ED138, a large trans-Neptunian object whose recovery and archival observations expose catalog gaps and suggest more bright objects remain undiscovered.

Question

The paper characterizes 2026 ED138 and establishes whether it is a real, previously unknown object, while investigating why earlier surveys and Rubin catalogs missed it. It also estimates how many trans-Neptunian objects with absolute magnitude H_V < 4 may remain undiscovered and where they may be found.

Method

The authors checked Rubin detections against catalogs, images, known-object records, and independent orbit fits, then extended the observing arc with DECam archival images. They fit the orbit using 74 observations spanning 3,789 days, measured photometry and colors, estimated size across assumed albedos, modeled volatile retention, and used recoveries of known bright TNOs to estimate the remaining population.

Findings

Result The adopted orbit is well constrained and places 2026 ED138 near the edge of Neptune’s 12:7 mean-motion resonance.
Authors’ interpretation Its absolute magnitude implies a diameter of 612–1530 km for geometric albedos from 0.25 to 0.04, making it a dwarf-planet candidate.
See the passage in the paper
Even the smaller of these sizes is above the size at which G. Tancredi & S. Favre [72] expect icy bodies to be close to hydrostatic equilibrium, about 450 km, so 2026 ED138 is a dwarf-planet candidate.
Authors’ interpretation The measured red color and dynamical comparisons indicate that 2026 ED138 is not a member of the Haumea collisional family.
See the passage in the paper
2026 ED138 is therefore not a family member.
Result Rubin’s direct-image catalogs omitted the object in 24 of 38 visits whose images showed it, primarily because crowded-field blends were not processed successfully.
See the passage in the paper
Because of this, as 2026 ED138 crosses a dense star field at low Galactic latitude Rubin’s single-visit catalogs have no entry for it in 24 of the 38 visits whose images show it (Table 4).
Authors’ interpretation The recovery-based estimate suggests about three further objects with H_V < 4 remain undiscovered outside the Galactic plane, with more likely near the plane and at greater distances.
See the passage in the paper
If the objects not yet discovered were as easy to find as the known ones, about 4 were undiscovered before the search (90% range 0.2–20), 2026 ED138 among them, so about 3 remain.

Limitations

The remaining-object estimate assumes undiscovered objects are as easy to find as known ones, although the authors expect the missed objects to be harder to detect.
See the passage in the paper
This estimate counts only what the Rubin search could have found, and it is probably an underestimate. First, still undiscovered objects are likely to be harder to find than known ones: they are the objects earlier surveys missed, because they are fainter in the sky, more distant, or in crowded fields, as 2026 ED138 was, and the same traits make them harder for Rubin.
The estimate covers the search’s measured efficiency away from the Galactic plane and does not count more distant objects.
See the passage in the paper
Second, the estimate applies only away from the Galactic plane.
The object's size and albedo remain uncertain because they have not been directly measured.
See the passage in the paper
Thermal photometry or a stellar occultation is needed to decide between these sizes.

Why it matters

The discovery demonstrates that bright TNOs can be missed even by deep surveys when they occupy crowded, poorly covered regions, and that Rubin’s early direct-image catalogs can also miss detections there. The authors argue that searches of the Galactic plane using image differencing, crowded-field processing, or image-based orbit searches should reveal additional objects.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: 2026 ED138; trans-Neptunian objects; 12:7 mean-motion resonance; Vera C. Rubin Observatory; Galactic-plane surveys

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Comprehensive Summary
Layman's Summary
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Original abstract

The paper reports the discovery and characterization of 2026 ED138, a large trans-Neptunian object (TNO) identified in early data from the Vera C. Rubin Observatory. It investigates how the object was verified and measured, why earlier surveys and Rubin’s own catalogs missed it, and how many other TNOs with absolute visual magnitudes below 4 may remain undiscovered. The team began with ten Rubin detections across eight nights, then searched additional Rubin observations and measured the object directly in images where the automated catalogs had not detected it. Local point-spread-function fits yielded 16 additional measurements, bringing the Rubin record to 30 observations over 129 days. To extend the orbit further back in time, the researchers searched archival Dark Energy Camera (DECam) images. They confirmed 44 detections across 14 nights between February 2016 and April 2023, using local astrometric calibration and checks against images of the same sky taken on other nights. Together, the Rubin and DECam data provide 74 observations spanning 3,789 days. Checks against known-object records, isolated tracklets, source catalogs, and the images support the conclusion that 2026 ED138 is real and had not been identified previously. Its measured barycentric orbit has a semimajor axis of 43.091 astronomical units, an eccentricity of 0.139, and an inclination of 24.43 degrees. The object lies near Neptune’s 12:7 mean-motion resonance, but its resonant status is uncertain. In 10-million-year integrations, 93 of 201 simulated orbits were resonant, a fraction of 0.46. This is near the authors’ threshold for a resonant candidate but well below the threshold for secure classification. The object appears to lie at the chaotic edge of the resonance. Although its orbit is near that of the Haumea collisional family, 2026 ED138’s red color (g–r = 0.63) argues against family membership. Its orbital properties also differ from those of confirmed family members. From Rubin photometry, the authors derive an absolute visual magnitude of 3.19. Its inferred diameter depends strongly on its unknown albedo: approximately 612 kilometres for an albedo of 0.25, 968 kilometres for 0.10, and 1,530 kilometres for 0.04. The authors consider diameters of roughly 612–968 kilometres more likely, while noting that thermal measurements or a stellar occultation are needed to determine its size. At these likely sizes, the object is a dwarf-planet candidate, not a confirmed dwarf planet. A simplified volatile-loss model predicts that 2026 ED138 would have lost methane, nitrogen, and carbon monoxide for albedos of 0.04 or greater. Only an unusually dark, larger body could retain some nitrogen or carbon monoxide. These predictions rely on simplified assumptions about atmospheric escape and do not fully account for processes such as impacts or resupply from the interior. The object escaped earlier searches because it lies far south and spent much of the relevant period near the crowded Galactic plane. Rubin’s direct-image catalogs also missed it in 24 of the 38 visits whose images show it, mainly because blends in crowded fields were not successfully processed. A comparison of Rubin’s recovery of known bright TNOs with its discovery of 2026 ED138 suggests that roughly three similar objects remain undiscovered outside the Galactic plane, with a broad 90% range of 0.2–20.

A Large, Elusive World at the Edge of the Solar System

Astronomers have identified 2026 ED138, a large trans-Neptunian object (TNO), in early observations from the Vera C. Rubin Observatory. The discovery is notable not only for the object’s size and unusual observing history, but also for what it reveals about the challenges of finding distant bodies in crowded regions of the sky.

The research paper, “2026 ED138: A Large Trans-Neptunian Object in Early Rubin Observatory Data, and How Many More May Remain,” is written by Matthew J. Holman, Mario Jurić, Kevin J. Napier, Jacob A. Kurlander, Aren Heinze, Joachim Moeyens et al.

Building a case from scattered observations

The team began with ten detections made by Rubin across eight nights. They then searched other Rubin observations and examined images directly, including visits where automated catalogs had not recorded the object. By fitting its point-like image in those frames, they extracted 16 additional measurements. In total, Rubin provided 30 observations over 129 days.

To extend the object’s recorded path further into the past, the researchers searched archival images from the Dark Energy Camera, or DECam. They confirmed 44 detections across 14 nights, from February 2016 to April 2023. Local checks of the astrometry and comparisons with images of the same sky from other nights helped establish that the detections were consistent.

Combined, the Rubin and DECam observations give 74 measurements spanning 3,789 days. The team also checked known-object records, isolated groups of detections called tracklets, source catalogs, and the images themselves. These checks support the conclusion that 2026 ED138 is a real object that had not previously been identified.

An orbit near Neptune’s resonance

The measured orbit has a semimajor axis of 43.091 astronomical units, an eccentricity of 0.139, and an inclination of 24.43 degrees. The object is near Neptune’s 12:7 mean-motion resonance, a relationship in which the objects’ orbital periods are close to a ratio of 12 to 7. But being nearby does not automatically mean that an object is securely trapped in the resonance.

To assess its status, the researchers integrated 201 possible orbits over 10 million years. Of these, 93 showed resonant behavior, a fraction of 0.46. That is near the team’s threshold for calling 2026 ED138 a resonant candidate, but well below the level needed for a secure classification. The object appears to sit at the resonance’s chaotic edge, where orbital behavior can be less stable.

Its orbit also falls near that of the Haumea collisional family, a group of objects thought to share a common origin. However, 2026 ED138’s red color, measured as g–r = 0.63, argues against family membership, as do differences between its orbit and those of confirmed family members.

How big is 2026 ED138?

Rubin photometry gives the object an absolute visual magnitude of 3.19. That brightness can imply very different sizes depending on how much sunlight the surface reflects. For example, the estimated diameter is about 612 kilometres at an albedo of 0.25, 968 kilometres at 0.10, and 1,530 kilometres at 0.04.

The researchers consider diameters of roughly 612 to 968 kilometres more likely, but the size remains uncertain. Thermal observations or a stellar occultation—when the object passes in front of a distant star—could help determine its diameter. At the likely sizes, 2026 ED138 is a dwarf-planet candidate, not a confirmed dwarf planet.

A simplified model of volatile loss suggests that the object would have lost methane, nitrogen, and carbon monoxide if its albedo is 0.04 or higher. A very dark, larger body might retain some nitrogen or carbon monoxide. These predictions are approximate: the model does not fully account for factors such as impacts or the possible resupply of material from the interior.

Why did earlier searches miss it?

2026 ED138 lies far south in the sky and spent much of the relevant period near the crowded Galactic plane, where dense fields of stars make moving objects harder to distinguish. Crowding also affected Rubin’s automated processing. The object appears in images from 38 Rubin visits, but the direct-image catalogs missed it in 24 of them, mainly because blends in those crowded fields were not successfully processed.

That gap between what an image contains and what an automated catalog records matters for estimates of the undiscovered population. Comparing Rubin’s recovery of known bright TNOs with its discovery of 2026 ED138, the team estimates that roughly three similar objects may remain undiscovered outside the Galactic plane. The uncertainty is broad: the estimated 90% range runs from 0.2 to 20 objects.

A discovery with a wider lesson

2026 ED138 shows how a distant object can remain hidden even in modern survey data—not because it is absent from the images, but because its location and surrounding stars make detection difficult. Its orbit, color, and uncertain size raise questions that future observations may answer. Just as importantly, the discovery suggests that systematic image-level searches could reveal other large TNOs overlooked by automated catalogs.

Created on 03 Nov. 2026

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Weekly letter Presented in "Astronomy and space this week" of 2 November 2026. Astronomy and space this week: Hidden worlds, stellar futures and the temperature of uncertainty

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2026 ED138: A Large Trans-Neptunian Object in Early Rubin Observatory Data, and How Many More May Remain

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

A Large, Elusive World at the Edge of the Solar System

Building a case from scattered observations

An orbit near Neptune’s resonance

How big is 2026 ED138?

Why did earlier searches miss it?

A discovery with a wider lesson

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