Dwarf planets beyond Pluto: ED138 and the science of distant, icy worlds

What the IAU says about dwarf planets—and Pluto’s reclassification

Historically, Pluto was counted among the nine classical planets. The 2006 IAU decision created a formal category for the largest subplanetary bodies that are not planets because they have not cleared their neighborhoods. In practice, this means a dwarf planet orbits the Sun, is nearly round, but shares its orbital region with other objects of similar size. The IAU also introduced the term plutoid to describe trans‑Neptunian dwarf planets, all of which orbit beyond Neptune. While some departments and scientists have debated the exact boundaries between planets and dwarf planets, the current framework is widely used in astronomy today.

Known dwarf planets and strong candidates

As a baseline, the nine largest bodies commonly discussed in this context include Ceres, Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, and Orcus. Ceres and Pluto are the best known; several of the others have moons that help determine their masses and densities, which in turn inform geophysical models of their interiors. The line between definitely confirmed dwarf planets and strong candidates is sometimes blurry: Salacia, for example, is a borderline case that remains under discussion. Beyond these, astronomers often mention additional large trans‑Neptunian objects—some of which are proposed as dwarf planets but have yet to receive full consensus.

How the Vera C. Rubin Observatory finds distant objects

The Vera C. Rubin Observatory is designed to survey the Southern Sky with unprecedented depth and speed. It features a massive 8.4‑meter mirror and the world’s largest digital camera, a 3.2‑billion‑pixel sensor, enabling it to image vast swaths of the sky repeatedly. Rubin can image the entire southern sky roughly every few nights, generating about 10 terabytes of data per night and revisiting each region hundreds of times over its planned decade‑long survey. Its software systems compare millions of detections across nights to identify moving objects—whether nearby asteroids or distant trans‑Neptunian bodies. This combination of speed, depth, and sky coverage makes Rubin especially powerful for discovering large, distant objects that previous surveys might have missed.

ED138: a potential sixth dwarf planet beyond Pluto

The object provisionally designated 2026 ED138 appears very large by trans‑Neptunian standards. Its diameter is estimated to fall between 600 and 1,500 kilometers (370 to 930 miles), which would place it above the size of the smallest confirmed dwarf planets like Ceres if the upper estimates hold. ED138 currently lies at about 46 astronomical units from the Sun, placing it well beyond Pluto and firmly in the outer Solar System. If confirmed as a dwarf planet, ED138 would join the five officially recognized dwarf planets—Pluto, Ceres, Haumea, Makemake, and Eris—and would be among the largest such bodies discovered in recent decades.

In the discovery narrative, a crucial human element helps readers connect with the science. Matthew Holman, a Smithsonian senior astrophysicist and lecturer at Harvard, developed a software pipeline to comb through Rubin commissioning data for moving objects. When ED138 emerged, his reaction was one of “excited disbelief.” He soon verified the signal and collaborated with Mario Jurić, Rubin’s Solar System lead scientist, to recover additional observations. Jurić’s search uncovered about 20 more sightings dating back to March 2026, and the team used data from other sources to trace the orbit back roughly ten years. This collaboration turned a single detection into a coherent orbital path, bolstering the case that ED138 is a real, distant world awaiting confirmation as a dwarf planet.

ED138’s discovery story is not just about one object. It underscores Rubin’s power to inventory the Solar System at new depths and speeds. The object’s location—in a crowded stretch of sky near the plane of the Milky Way—help explains why it wasn’t found earlier. The dense star fields in that region complicate detection of faint, slow‑moving objects. Holman notes that the plane of the Milky Way and the southern position of many telescopes contributed to the late discovery, a point echoed by observers who emphasize Rubin’s unique capabilities in such challenging regions. The object’s southern orientation further limited northern‑hemisphere search programs, illustrating how telescope coverage shapes what we can detect and when.

Beyond these specifics, the ED138 announcement has broad implications. Rubin’s commissioning observations already yielded thousands of new Solar System objects—more than 11,000 asteroids, including approximately 380 beyond Neptune—highlighting Rubin’s potential to transform our understanding of the outer Solar System and to reveal objects that challenge or refine the dwarf‑planet criteria. As one Rubin team member put it, the survey’s depth and repeated sky coverage will allow researchers to uncover objects hidden by density of stars, telescope geometry, or historical observing gaps.

Current assessment is that ED138 remains a candidate. The IAU and the wider community will consider follow‑up observations to measure its size more precisely, assess its shape, and search for a moon, all of which would help determine its mass and density. Infrared observations (potentially with the James Webb Space Telescope) could constrain albedo and size, while stellar occultations and moon detections would tighten mass and structure estimates. A definitive determination will require this multi‑method confirmation work, but ED138 has already demonstrated Rubin’s ability to reveal large, distant worlds that reshape the outer Solar System landscape.

One more context point helps readers appreciate the scale of ED138. If its diameter is at the upper end of the current range, it could rival Makemake, and it might be the largest trans‑Neptunian object found in roughly two decades. Even at the lower end, ED138 remains among the larger bodies discovered in the outer Solar System in recent years, illustrating the ongoing pace of discovery as surveys push toward the edge of the planetary realm.

As researchers continue to refine ED138’s status, a few related questions naturally arise. The object’s orbital tilt—about 25 degrees from the ecliptic—classifies it as a hot classical trans‑Neptunian object, a population believed to have been dynamically stirred by Neptune’s migration early in the Solar System’s history. The label “hot” does not refer to temperature; it signals an unusually inclined and somewhat elongated orbit compared with the cold classical population. This dynamical history helps explain why ED138’s orbit remains stable today while preserving clues about the Solar System’s past.

Looking ahead, the scientific payoff of ED138 depends on continued observations. Infrared measurements will help pin down its size and surface properties; a stellar occultation campaign could reveal its exact silhouette, and a discovered moon would enable a direct mass measurement. Together, these data would sharpen density estimates and illuminate the object’s composition and interior structure, offering a window into the processes that shape distant icy worlds.

Why discoveries like ED138 matter—and what comes next

Edging beyond Pluto, each new discovery expands the catalog of large trans‑Neptunian objects and tests the boundaries of the IAU’s dwarf‑planet definition. The most widely cited major bodies—Ceres, Pluto, Eris, Haumea, Makemake—represent a core set known to have reached hydrostatic equilibrium or to be close to it, with many others occupying a gray area where albedo, composition, shape, and mass remain uncertain. The discovery of ED138, and the ongoing effort to confirm its status, illustrates how modern surveys combine deep imaging, advanced data processing, and cross‑telescope collaboration to reveal objects that previously lay hidden in dense star fields and far from Earth’s best vantage points.

Beyond confirming a single object, Rubin’s results—thousands of newly found Solar System bodies from the inner asteroid belt to the far reaches beyond Neptune—signal a broader shift: the outer Solar System census is rapidly expanding, and our understanding of how dwarf planets form, evolve, and interact with their environments will continue to evolve as more data arrive from infrared follow‑ups, occultations, and potential moon discoveries. For readers, this means a living, evolving picture of the Solar System—one that keeps revising which bodies are planets, which are dwarf planets, and which worlds remain to be discovered.

Key terms to guide your reading

In this area of planetary science, several terms recur. A dwarf planet is a planet‑mass body that orbits the Sun, is roughly round, but has not cleared its neighborhood. A plutoid is a trans‑Neptunian dwarf planet; the IAU formally introduced the term in 2008, though some departments have resisted its broader acceptance. The debate about how best to define a planet—whether to emphasize dynamic dominance (clearing nearby objects) or intrinsic properties (roundness and geology)—drives ongoing discussions about what objects count as planets versus dwarf planets. Another category sometimes encountered is the hot classical trans‑Neptunian object, describing a subset of the Kuiper belt with inclined orbits likely sculpted by Neptune’s early migration. These terms help frame the current scientific landscape as researchers refine classification in light of new discoveries.

What observations will settle ED138’s status?

To confirm ED138 as a dwarf planet, astronomers will pursue infrared follow‑ups (potentially with the James Webb Space Telescope) to improve size and albedo estimates. They will also look for stellar occultations, where the object passes in front of a background star, yielding precise size measurements. The possible detection of a moon would enable a dynamical mass measurement, tightening density estimates. Each of these steps narrows the uncertainties about ED138’s nature and helps place it within the broader context of the Solar System’s largest icy worlds.

Naming, debate, and the ongoing classification conversation

As with other large trans‑Neptunian candidates, ED138 will eventually receive a permanent designation and a formal name if confirmed. The naming process sits at the intersection of scientific discovery and community tradition, reflecting both the object’s intrinsic nature and the symbolic role these distant worlds play in our exploration of the Solar System. Some names proposed for large dwarf planets have been taken or reserved, underscoring the community’s evolving, collaborative approach to nomenclature.

In this evolving landscape, Máni has been discussed as a debated candidate in some circles. Consensus on Máni’s status has varied in recent literature, and readers should watch for updates from the IAU and major surveys as evidence about hydrostatic equilibrium and shape solidifies. The ED138 case illustrates how new data can challenge or reinforce long‑standing perspectives on which bodies deserve a planetary designation.

A final perspective

ED138 exemplifies a broader trend: technology, collaboration, and careful interpretation are expanding our map of the outer Solar System. Rubin’s capabilities, combined with infrared follow‑ups and occultations, hold the promise of revealing not just one object, but a richer understanding of how large, icy worlds form, evolve, and interact with Neptune’s past migrations. For readers, the message is clear: the Solar System still contains major discoveries waiting in the dark, and the process of discovery—driven by modern surveys and human curiosity—continues to reshape our view of what a planet, and what a dwarf planet, can be.