TL;DR
Astronomers identified 2026 ED138 in Vera C. Rubin Observatory data and traced it in earlier 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; its size, and whether it qualifies as a dwarf planet, remain uncertain.
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 confirmed in observations spanning decades, but researchers still need to measure its size before they can determine whether it meets that classification.
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Smithsonian Magazine reported November 12 that Matthew Holman, a senior astrophysicist at the Smithsonian Astrophysical Observatory, spotted the object while examining data from the Vera C. Rubin Observatory in Chile. Holman and colleague Kevin Napier had used a custom computer program to identify more than 2,000 small, distant solar system objects in the data. ED138 stood out as a possible larger body.
The research team checked the apparent discovery against older records, working with Napier and Rubin solar system lead scientist Mario Jurić. They found the object in images taken over the past decade and in photographic plates from 1937 held at Harvard. According to Holman, the repeated appearances at its predicted position confirmed that it is a real solar system object, rather than an imaging artifact.
ED138 is about 46 times farther from the Sun than Earth, and its orbit is tilted nearly 25 degrees relative to the plane followed by the major planets. The report says that tilt may reflect gravitational interactions with Neptune. Current size estimates range from 370 to 930 miles across, a wide span that is not sufficient to settle its classification.
A Possible Addition to the Dwarf Planets
If measurements show that ED138 is large enough for its gravity to have pulled it into a nearly round shape, it could join the recognized dwarf planets. The category currently includes Pluto, Eris, Makemake, Haumea and Ceres. Dwarf planets also have not cleared their orbital neighborhoods of other similar-size debris, unlike the solar system’s eight major planets.
The potential discovery matters because distant, faint objects can reveal parts of the solar system that are difficult to study. ED138’s long orbit and large distance mean that observations are challenging, and its unusual orbital tilt may offer evidence about how its path was shaped. Still, the object’s existence does not by itself establish that it is a dwarf planet, nor does it show how many similar bodies remain undiscovered.
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Why Size Determines Its Status
A dwarf planet must orbit the Sun and be massive enough for its own gravity to give it a nearly round shape, while remaining too small to clear other objects from its orbital region. ED138 appears bright in telescope images, but brightness alone cannot establish size: a smaller body with a highly reflective surface could appear as bright as a larger, darker one.
Astronomers have long expected that additional dwarf planets may exist beyond those already recognized. Some distant bodies spend much of their time on elongated orbits far from the Sun, making them hard to detect. Holman described known objects as potentially the “tip of the iceberg” of bodies seen when they are closer along their orbits. The report also notes that searches face extra difficulty in sky regions crowded with background light from the Milky Way.
“We know that it’s real. Every time you look at the predicted location, there it is.”
— Matthew Holman, senior astrophysicist at the Smithsonian Astrophysical Observatory
The Object’s Size Is Still Unknown
The central unresolved question is whether ED138 is large enough to be round under its own gravity. Its estimated diameter spans 370 to 930 miles, and the report says brightness alone cannot narrow that range reliably because surface reflectivity affects how bright an object looks. The dwarf planet label remains a possibility, not a confirmed classification.
Researchers may measure the object in infrared light, which can help estimate its size from the heat it emits after absorbing sunlight. Another option is to observe ED138 as it passes in front of a star; the duration of the dimming, combined with the star’s light and the object’s speed, can help calculate its diameter. The report says astronomers are working toward such observations, including with the Atacama Large Millimeter/submillimeter Array. It does not give a completed measurement or a final classification.
Observations to Pin Down Diameter
The next step is to obtain observations that can distinguish a large, relatively dark body from a smaller, more reflective one. Holman said ED138 is not especially easy to see at present, but that it should become more accessible in the coming months, when astronomers will try to observe it. Infrared measurements or a star-occultation observation could provide the size evidence needed for a classification.
Until those observations are reported and interpreted, ED138 should be described as a dwarf planet candidate. The source report does not specify a date for a final size estimate, and the candidate’s precise physical properties remain open.
Key Questions
What is 2026 ED138?
It is a distant solar system object identified in Vera C. Rubin Observatory data and confirmed in older images, including a photographic plate from 1937. Astronomers are investigating whether it is large enough to be a dwarf planet.
How long does ED138 take to orbit the Sun?
The reported orbital period is 281.1 years. It is about 46 times farther from the Sun than Earth.
Has ED138 been confirmed as a dwarf planet?
No. The team has confirmed that the object is real, but its size has not been established. Its dwarf planet status depends in part on whether it is large enough for its gravity to make it nearly round.
How will astronomers measure its size?
They may observe its infrared heat emission or measure how long it blocks a star’s light as it passes in front of one. The report says astronomers are working on observations, including with ALMA, but does not give a result yet.
Source: smithsonianmag.com
