Back in 2006, the International Astronomical Union controversially demoted Pluto.
This composite image of Pluto and its largest moon, Charon, was based on photographs taken by the New Horizons mission as it flew by the Plutonian planetary system back in 2015. Charon’s appearance is vastly different from Pluto’s, but both bodies are shown with the correct relative size and albedo. The surface features on Pluto show evidence of recent changes, indicating that the next time we visit Pluto to image it at this level of precision, it may exhibit a remarkably different appearance.
As part of that process, they officially defined “planet” and “dwarf planet” for the first time.
The scientific line between planetary (above) and non-planetary (below) status, for three potential definitions of an orbit-clearing phenomenon and a star equal to the mass of our Sun. This definition could be extended to every exoplanetary system we can imagine to determine whether a candidate body meets the criteria, as we’ve defined them, for being classified as a true planet or not.
Dwarf planets all:
- orbit the Sun,
- aren’t moons,
- achieve hydrostatic equilibrium,
- but don’t clear their orbits.
The four largest asteroids, all shown here, have been imaged with NASA’s Dawn mission and the ESO’s SPHERE instrument. Ceres, the largest asteroid, is the smallest known body definitively in hydrostatic equilibrium. Vesta and Pallas are not, but Hygeia, with a smaller mass but a much lower density, may yet be; its status is indeterminate.
Therefore, sufficiently massive asteroids like Ceres,
Under a size cutoff of 10,000 kilometers, objects within our solar system appear to be round, pulled into hydrostatic equilibrium via their gravity and rotation, combined. However, once you go to planetary radii below ~800 kilometers, hydrostatic equilibrium, or even roundness, are no longer certainties, but composition dependent. There are over 100 objects in hydrostatic equilibrium in our solar system, including asteroids, moons, planets, dwarf planets, and Kuiper belt and Oort cloud objects, but only 5 official dwarf planets as of 2026.
Credit: Emily Lakdawalla; data from NASA/JPL, JHUAPL/SwRI, SSI, and UCLA/MPS/DLR/IDA
Kuiper belt objects like Pluto, Eris, Makemake, and Haumea,
The inner Solar System, including the planets, asteroids, gas giants, Kuiper belt, and more, is minuscule in scale when compared to the extent of the Oort Cloud. Sedna, the only large object with a very distant aphelion, may be part of the innermost portion of the inner Oort Cloud, but even that is disputed. On a linear scale, depicting the entire Solar System in a single image is incredibly limiting; to characterize the orbit of a faraway bound object requires years or even centuries of data.
and Oort cloud objects can achieve dwarf planethood.
This chart shows the measured pressures (or constraints on pressure) around a variety of trans-Neptunian objects. Of all the objects ever measured through stellar occultations, only Pluto and 2002 XV93 were found to actually have non-zero atmospheres, with all other plutinos, including plutinos much larger than 2002 XV93, exhibiting no signature of an atmosphere at all.
Trans-Neptunian objects, ice-rich in composition, require estimated diameters of 400 km to likely achieve hydrostatic equilibrium.
This image shows the Vera C. Rubin Observatory atop Cerro Pachón in Chile, with fairly typical cloud-free conditions accompanying it. When complete, Rubin Observatory will use its 8.4-meter telescope and 3200-megapixel camera to conduct an unprecedented, multi-color, decade-long survey of the optical sky called the Legacy Survey of Space and Time (LSST). The camera will be the largest ever constructed for astronomy, weighing an incredible 2.8 tonnes.
Currently, the Vera C. Rubin Observatory surveys the visible southern skies every 3-4 nights.
This image, taken in April of 2025, shows the completed and operational Vera C. Rubin Observatory with its dome open during its First Look observation activities. Overhead, the Beehive Cluster (Messier 41) shines bright, while below, the glow of nearby small cities shines in this mountainous landscape.
Through successive imagery, it uncovers faint, distant, migrating Solar System objects.
This selection of 10 independent images of the same object, 2026 ED138, shows its migration relative to the backdrop of stars in the galactic plane. Only with Vera C. Rubin’s unique sensitivity and cadence can we discover objects this faint and distant in such a crowded field of view.
Helped by automated code, Rubin scientists identified a remarkable object 2026 ED138.
“Precovery” images were spotted going back 10 years, enabling an orbital determination.
This image shows the orbital pathway of newly discovered candidate dwarf planet 2026 ED138, as observed by the NSF’s Vera C. Rubin Observatory. The position at the start of each month between March and July of 2026 is highlighted with annotations. The orbital path is easily reconstructible based on the observed motion and rate of progression of the object across the sky, allowing us to determine its distance and status as a classical TNO.
Based on its distance, brightness, and estimated reflectivity, it’s definitely a Trans-Neptunian object.
This top-down view of the Solar System shows the first five officially recognized dwarf planets, Ceres, Pluto, Eris, Makemake, and Haumea, along with the newest dwarf planet candidate: 2026 ED138. Observationally, 2026 ED138 is the 14th brightest TNO to date, but its physical size dimensions possess great uncertainties.
Its estimated size — between 600 km and 1500 km — places it firmly in dwarf planet territory.
Over the course of just one month, the newly discovered candidate dwarf planet 2026 ED138 has migrated across this entire field of view of the Vera C. Rubin observatory. Circled locations reveal instances where the telescope captured this Kuiper belt object migrating against the fixed background of stars.
However, no new dwarf planets have been officially recognized by the IAU since 2006.
This composite shows the 10 largest known Trans-Neptunian objects as of 2017, all classified as extremely likely to be dwarf planets. However, the IAU has officially recognized only 5 total worlds as dwarf planets as of 2026: Ceres, Pluto, Eris, Makemake, and Haumea; the list has not been updated in over 20 years.
2026 ED138, nicknamed “Ed“, becomes the 103rd known Trans-Neptunian object 400+ km in diameter.
The 14th brightest Trans-Neptunian object, future occultations should reveal its size.
When one astronomical object occupies the same line-of-sight as another, an occultation will occur, as the “closer” object blocks the light that would otherwise be visible from the “farther” object. The Moon occults all of the other planets; the Moon and planets and other planetary bodies occult background stars, revealing the relative distances between them. If a planetary body has a substantial atmosphere, occultation studies can reveal that as well, which is how Pluto’s atmosphere was discovered back in the late 1980s. The expectation is that 2026 ED138’s possible atmosphere, or constraints on it, will be revealed by future occultation studies.
Credit: Bob King/Stellarium/Sky & Telescope
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This article What Vera Rubin observatory’s first new dwarf planet means is featured on Big Think.