Dwarf Planet

Astronomers have identified a distant object beyond Pluto that could become the Solar System’s sixth officially recognised dwarf planet. Provisionally designated 2026 ED138, the icy body was detected in observations from the NSF–DOE Vera C. Rubin Observatory in Chile. Early estimates suggest that it may be between 600 and 1,500 kilometres in diameter, potentially placing it among the largest known objects in the distant outer Solar System.

The discovery is attracting scientific attention because objects of this size can provide important clues about how the Solar System formed and evolved. However, researchers have not yet confirmed that ED138 meets all the requirements to be classified as a dwarf planet. Further observations are needed to establish its size and determine whether its own gravity has shaped it into a roughly spherical form.

What Is 2026 ED138?

2026 ED138 is a trans-Neptunian object, meaning it travels around the Sun in the distant region beyond Neptune’s orbit. Its estimated distance is approximately 46 astronomical units from the Sun. One astronomical unit is the average distance between Earth and the Sun, approximately 150 million kilometres.

For comparison, Neptune orbits the Sun at an average distance of about 30 astronomical units, while Pluto’s average distance is roughly 39.5 astronomical units. ED138 therefore occupies a region of the Solar System where sunlight is weak and temperatures are extremely low.

Scientists believe the object is likely to contain substantial amounts of ice, as many bodies in the outer Solar System do. Its exact composition, surface characteristics and physical dimensions remain uncertain.

The estimated diameter range of 600 to 1,500 kilometres is based on its observed brightness, distance and assumptions about how much sunlight its surface reflects. A highly reflective object can appear brighter than a darker object of the same size, so brightness alone cannot establish its precise dimensions.

If additional observations confirm that ED138 is sufficiently large and rounded by its own gravity, it could qualify as a dwarf planet. Until then, it remains a candidate rather than a confirmed member of that category.

How Did Astronomers Find the Object?

The discovery emerged from the analysis of observations collected by the Vera C. Rubin Observatory earlier in 2026. On October 13, astronomer Matthew Holman of the Center for Astrophysics | Harvard & Smithsonian was examining data using a computer program developed with colleague Kevin Napier.

The software is designed to search through enormous numbers of detections and identify patterns that reveal moving objects in the sky. Most distant objects appear as faint points of light, making them difficult to distinguish from the far greater number of stars and other background sources.

The search identified ED138 as an unusually bright object at a considerable distance from the Sun. Its apparent brightness suggested that it might be substantially larger than many other bodies found in the same region.

Researchers then checked the result against additional observations to make sure the detection was genuine. The team found that the Rubin Observatory had recorded the object 30 times across 20 nights between March 4 and July 11, 2026.

Astronomers also located older observations from other facilities, including archival images dating back to 2016. These records helped scientists reconstruct the object's movement over a longer period and improve their understanding of its orbit.

Finding older images of an object after its discovery is particularly valuable in astronomy. It allows researchers to extend the observational record without waiting years for new measurements. A longer record can improve orbital calculations and help determine where a distant object will appear in the sky in the future.

Why Was It Not Discovered Earlier?

One reason ED138 remained unidentified is the crowded region of the sky in which it was observed. The object lies near the plane of the Milky Way, where numerous stars and other sources of light can make faint moving bodies difficult to detect.

Traditional searches can struggle to distinguish a slowly moving object from the dense background of stars. A telescope may record the object without immediately revealing that the same point of light has shifted between observations.

The Rubin Observatory is designed to address this challenge by repeatedly surveying the sky and collecting large volumes of high-resolution data. Its powerful camera and automated analysis systems help scientists identify changes that might otherwise go unnoticed.

Rather than relying on a single image, researchers can compare observations taken on different dates and look for consistent movement. Combining those observations with archival data can make it possible to identify objects that were present in earlier images but were not recognised at the time.

ED138 demonstrates how improved data analysis can uncover objects that are already within existing astronomical records. The discovery does not necessarily mean the object recently entered the Solar System; it means scientists have now identified and tracked it as a distinct body.

What Makes a Dwarf Planet Different From a Planet?

A dwarf planet is a body that orbits the Sun and has enough mass for its own gravity to pull it into a nearly round shape, but has not cleared the neighbourhood around its orbit of other objects.

This distinction became widely known in 2006, when the International Astronomical Union adopted a formal definition of a planet that resulted in Pluto being classified as a dwarf planet.

The five dwarf planets recognised by the International Astronomical Union are Pluto, Ceres, Eris, Haumea and Makemake. Ceres is located in the asteroid belt between Mars and Jupiter, while the other four lie in the outer regions of the Solar System.

ED138 would potentially become the sixth member of this recognised group if its physical characteristics satisfy the relevant criteria and its status is formally accepted.

Its possible size makes it particularly interesting. A body several hundred kilometres across may have experienced enough internal gravitational pressure to become rounded, but scientists need more precise measurements before drawing a firm conclusion about this object.

What Happens Next?

The next stage involves observations capable of narrowing down ED138’s physical size. Infrared measurements are especially useful because astronomers can study the heat emitted by an object, helping them estimate its dimensions more accurately.

The difficulty is that ED138 is extremely distant and faint. Obtaining reliable measurements will require sensitive instruments and careful analysis. Researchers also need to refine its orbital information so that future observations can target it accurately.

Confirmation could take time, and the object’s provisional designation should not be mistaken for an official name or a final classification.

If ED138 is confirmed as a dwarf planet, it will add a significant new member to the known population of large bodies beyond Neptune. If it does not meet the criteria, it will still remain an important discovery and a useful subject for studying the outer Solar System.

Why This Discovery Matters

The potential discovery highlights how much remains unknown about the Solar System’s distant outskirts. Beyond Neptune lies a vast population of icy objects left over from the early stages of planetary formation. Studying these bodies can help researchers investigate how material accumulated, how planetary gravity influenced smaller objects and how the Solar System developed over billions of years.

The discovery also illustrates the value of long-term astronomical surveys. The Rubin Observatory is beginning a decade-long programme to repeatedly image the southern sky, creating a growing record of objects that change position or brightness over time.

As researchers develop better methods to analyse these observations, additional distant objects may emerge from data that would once have been too difficult to examine in detail.

The key point is that 2026 ED138 is a confirmed astronomical object but only a candidate dwarf planet. Its estimated size makes it an exciting discovery, yet further observations must establish its physical properties before its final classification can be determined.