Frozen object in outer Solar System

A frozen object with a diameter between 600 and 1,500 kilometers, designated 2026 ED138, has been identified in images from the Vera C. Rubin Observatory in Chile. If it is massive enough to have taken on a rounded shape, it could become the sixth recognized dwarf planet in the Solar System.

Discovery by astrophysicist

Matthew Holman, an astrophysicist at the Center for Astrophysics of Harvard and the Smithsonian, discovered 2026 ED138 by analyzing data from the Rubin Observatory, located on Cerro Pachón. Holman and his colleague Kevin Napier developed a program to review billions of detections of moving objects and identify those belonging to the same celestial body.

Unusual brightness and orbit

Applying it to images collected by Rubin since the beginning of the year, they noticed an object too bright for its distance, a sign of an unusual size. The object orbits the Sun at 46 astronomical units, which is 46 times the distance between Earth and the Sun, while Pluto averages around 40 astronomical units.

Potential dwarf planet status

If it passes the examination, it will join Pluto, Ceres, Eris, Haumea, and Makemake, the five dwarf planets recognized by the International Astronomical Union (IAU). At this distance, 2026 ED138 receives about 2,000 times less sunlight than Earth and moves very slowly across the sky, near the plane of the Milky Way, where millions of stars can be counted.

Verification of object’s presence

Mario Jurić, a professor at the University of Washington and principal scientist at the Rubin Solar System Center, verified the object's presence in 30 images taken over 20 nights. Using the ADAM Precovery service of the Asteroid Institute, the team also found the object on archived images from the DECam camera, installed on a telescope at the Cerro Tololo Observatory in Chile, and reconstructed more than ten years of its orbit, back to 2016.

Orbital characteristics and gravitational influence

Its orbit is inclined by nearly 25 degrees relative to the plane of the Solar System, and researchers believe that Neptune has deviated its initial trajectory through gravitational attraction. 2026 ED138 is the first candidate for the dwarf planet title discovered by Rubin, in data collected even before the official launch of its large sky survey.

Size estimation and infrared observations

No one has yet directly measured the size of 2026 ED138, and astronomers deduced this range from its distance, brightness, and an estimate of its reflectivity. In visible light, a small body covered with highly reflective ice and a large body with a dark surface shine almost equally from Earth. Researchers will need to conduct observations in the infrared to determine its actual size, because a dark surface absorbs more sunlight and radiates more heat.

Rubin Observatory’s long-term mission

For ten years, the Rubin teams will photograph the entire southern sky every three days or so with the largest digital camera ever built, to which several laboratories of the CNRS have contributed. Matthew Holman is counting on these repeated passes to uncover other objects of this size, hidden in the areas that previous surveys had overlooked.

Precovery and orbit accuracy

Precovery consists of finding an object already present on archive images taken before its official identification. By adding these older positions to those measured recently, the observation arc is significantly extended, which reduces uncertainties about the orbit. This is particularly useful beyond Neptune, where the apparent movement is very slow and where a few weeks of data can leave large ambiguities.

Dwarf planet criteria and IAU recognition

A dwarf planet must orbit the Sun and not be a satellite of a planet. It must also have sufficient mass to achieve hydrostatic equilibrium, thus a generally spherical shape, which depends on its size and composition (ice or rock). Finally, it has not "cleared" the neighborhood of its orbit, unlike the eight planets that dominate gravitationally in their zones. Official recognition remains a decision of the International Astronomical Union, and not all candidates proposed by the community immediately appear on the list.