TL;DR
Astronomers identified 2026 ED138 in Vera C. Rubin Observatory data and traced it in archival images, including photographic plates from 1937. It orbits about 46 times farther from the Sun than Earth and takes 281.1 years to complete an orbit, but its size has not been measured well enough to confirm it as a dwarf planet.
Astronomers have identified 2026 ED138, a distant object that takes 281.1 years to orbit the Sun and may be large enough to qualify as a dwarf planet. The object has been found in images spanning decades, but its size is still uncertain, so its classification has not been established.
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Matthew Holman, a senior astrophysicist at the Smithsonian Astrophysical Observatory, spotted the object while examining data from the Vera C. Rubin Observatory in Chile. He and colleagues Kevin Napier and Rubin solar system lead scientist Mario Jurić checked earlier observations to verify that the object was real and to refine its orbit. Holman told Smithsonian magazine that it appears at the location predicted from its orbit each time researchers look. The team also located it in photographic plates from 1937.
2026 ED138 is about 46 times farther from the Sun than Earth, according to the report, and its orbital plane is tilted by nearly 25 degrees relative to the plane followed by the major planets. That tilt may point to past gravitational interactions with Neptune, but it does not by itself establish the object’s origin or classification.
Current estimates put the body’s diameter somewhere between 370 and 930 miles. That range is too broad to settle whether it is large enough for gravity to have made it nearly round, a key feature of dwarf planets. Brightness alone cannot determine its size: a small, highly reflective object could look as bright as a larger, darker one.
A New Object Beyond the Known Dwarfs
If measurements show that 2026 ED138 is large enough to be nearly round, it could join a small recognized group: Pluto, Eris, Makemake, Haumea and Ceres. Dwarf planets orbit the Sun and are rounded by their own gravity, but have not cleared other similarly sized bodies from their orbital paths. The distinction is part of why Pluto was reclassified in 2006.
The discovery also adds evidence that the distant solar system may contain objects that have escaped detection. Holman told Smithsonian magazine that observed objects may be the “tip of the iceberg” of a larger population. That possibility remains an inference, not a count of undiscovered bodies: objects on long, distant orbits can be difficult to see, and their visibility changes as they move around the Sun.
The orbit and possible size make ED138 scientifically useful even before its status is resolved. Better measurements could help researchers understand the range of bodies beyond Neptune and how their paths may have been shaped by gravitational encounters. The discovery does not, on its own, confirm a new dwarf planet.
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How the Candidate Was Verified
The object first stood out among more than 2,000 small bright points identified in Rubin Observatory data by Holman and Napier’s computer program. Distant-object searches can produce false positives: Holman told Smithsonian magazine that astronomers sometimes think they have found something unusually large, only for closer examination to show otherwise. The team therefore checked the apparent object against earlier sky images before drawing conclusions.
Finding it in observations taken over many years—including the 1937 Harvard photographic plates—gave researchers positions at different times. Those observations helped establish that the object follows a consistent path rather than being an image artifact, and helped determine its unusually long orbit. The report says Rubin captured it in images made between March and June 2026.
Only five objects are currently recognized as dwarf planets in the source report: four in the distant Kuiper Belt and Ceres in the asteroid belt between Mars and Jupiter. ED138 is a candidate, not an addition to that recognized list. Its measured orbit and uncertain diameter are distinct questions: the former is supported by observations, while the latter still needs follow-up.
““We know that it’s real.””
— Matthew Holman, senior astrophysicist at the Smithsonian Astrophysical Observatory
Size Still Determines Its Status
The main unresolved issue is whether 2026 ED138 is large enough to be nearly round. Its estimated diameter spans 370 to 930 miles, and its visible brightness cannot distinguish reliably between a smaller reflective surface and a larger, less reflective one. Until researchers obtain better size data, calling it a dwarf planet would go beyond what has been confirmed.
Astronomers can estimate its size by measuring the heat it emits in infrared light or by observing it pass in front of a star. In the latter method, the duration of the star’s dimming, combined with the star’s brightness and the object’s motion, can help determine the body’s dimensions. The report says researchers are working toward observations, including with the Atacama Large Millimeter/submillimeter Array.
The object is not especially easy to observe at present, according to Holman, and the report does not give a final measurement or a confirmed date for the needed observations. The possible role of Neptune in tilting its orbit is also an interpretation of the orbit, not a demonstrated account of its history.
Follow-Up Observations Ahead
Researchers are expected to pursue observations as 2026 ED138 becomes easier to see in the coming months, Holman told Smithsonian magazine. Infrared measurements or a stellar occultation could narrow the size estimate and clarify whether the object is physically large enough to be rounded by its own gravity.
Until those observations produce a more precise diameter, the appropriate description remains dwarf planet candidate. The source report does not provide a scheduled observation date, a result from follow-up measurements or a formal classification decision. Further work may also refine the orbit as new observations are added.
Key Questions
What is 2026 ED138?
2026 ED138 is a distant solar system object identified in Vera C. Rubin Observatory data and confirmed in earlier images. Astronomers are investigating it as a possible dwarf planet, but that classification has not been confirmed.
How long does it take to orbit the Sun?
Its estimated orbital period is 281.1 years. It is about 46 times farther from the Sun than Earth.
Why is it not yet confirmed as a dwarf planet?
A dwarf planet must be nearly round, but researchers do not yet know ED138’s size precisely enough to determine whether it meets that condition. Its brightness alone cannot settle the question.
How will astronomers measure its size?
They can study the heat it emits in infrared light or measure how long it blocks a star during a pass in front of it. The report says astronomers are working on follow-up observations, including possible use of ALMA.
Has the object only just appeared?
No. It drew attention in recent Rubin Observatory data, but researchers found it in older sky images, including photographic plates dating to 1937. Those archival detections helped verify the object and establish its orbit.
Source: smithsonianmag.com
