Astronomers using the Vera C. Rubin Observatory in Chile have identified a large object beyond Pluto that may qualify as a dwarf planet, adding a potentially significant member to the outer solar system’s growing population of candidates. Designated ED138, the object is estimated to measure between 600 and 1,500 kilometers across.
That size places ED138 near the threshold commonly used to distinguish a dwarf planet from a large asteroid. An object must be massive enough for its own gravity to pull it into a roughly spherical shape, but determining the mass and shape of distant bodies directly is extremely difficult. Astronomers generally accept that objects larger than about 600 kilometers in diameter are likely to meet that physical condition.
A large object hidden near the Milky Way
ED138 was difficult for earlier observatories to detect because it lies close to the plane of the Milky Way. The dense field of bright background objects in that region can obscure comparatively faint bodies in the outer solar system. The Rubin Observatory’s combination of survey speed, depth and precision allowed researchers to identify the object and then look for it in earlier observations.
“Rubin is giving us an unprecedented capacity to explore the sky through a combination of speed, depth, and precision,” said Mario Jurić of the University in Washington.
Jurić said the initial identification is only the beginning of the process. After Matthew Holman of the Harvard-Smithsonian Center for Astrophysics first spotted ED138 in October, Jurić searched Rubin’s earlier catalog and located 30 additional observations dating back to March. Those observations provide a longer record of the object’s movement, which is important for determining its orbit and refining estimates of its properties.
What would make ED138 a dwarf planet?
The International Astronomical Union currently recognizes five official dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. Other distant bodies, including Sedna and Quaoar, are widely treated as unofficial members of the broader category. Given its estimated size, ED138 appears likely to join that unofficial group, although the designation described by astronomers remains a candidate status rather than a formal addition to the IAU’s list.
- Object: ED138
- Estimated diameter: 600 to 1,500 kilometers
- Location: Beyond Pluto, near the plane of the Milky Way
- Observatory: Vera C. Rubin Observatory in Chile
- Earlier records found: 30 observations dating back to March
The discovery also illustrates why archival searches are an important part of modern astronomy. Once researchers know where to look, images collected months or even decades earlier can reveal a faint object that was not recognized at the time. Those additional sightings help establish an orbit and can turn a single observation into evidence for a persistent body in the solar system.
ED138’s eventual classification will depend on further analysis of its orbit, size and physical characteristics. For now, its estimated dimensions make it one of the largest dwarf-planet candidates identified beyond Pluto, while the Rubin Observatory’s expanding survey is expected to provide more opportunities to find similarly faint objects that earlier facilities missed.
