is an unnamed trans-Neptunian object in the scattered disk, orbiting the Sun on a highly distant and elliptical orbit. It is in a 2:5 orbital resonance with Neptune, meaning its orbital period is exactly times that of Neptune's. It was discovered at Palomar Observatory on 9 October 2002, during a search for trans-Neptunian objects by Chad Trujillo, Michael E. Brown, and the Near-Earth Asteroid Tracking program. The object measures approximately in diameter and has a shape resembling a flattened sphere, making it a possible dwarf planet.
has a cold and icy surface made of frozen water, carbon dioxide, carbon monoxide, methanol, and various complex organic compounds. These complex organic compounds, called tholins, color the object's surface red and are thought to be byproducts of ice irradiated by solar and cosmic rays. rotates slowly, with a rotation period of approximately 56 hours, and may possess a large unresolved moon based on indirect evidence from telescope observations.
Observational history Discovery was discovered on 9 October 2002 by Palomar Observatory in California, United States. At the time, Chad Trujillo and Michael E. Brown were searching for trans-Neptunian objects using the observatory's 1.2-meter Samuel Oschin telescope. Their search had been operating jointly with the Near-Earth Asteroid Tracking (NEAT) program, which contributed to the discovery of using the same telescope. The discovery of was announced by the Minor Planet Center (MPC) on 7 November 2002, after several observatories reobserved the object during that month.
When the discovery of was announced, its orbit was still poorly constrained due to the small number of observations recorded. This was resolved on 11 December 2002, when the MPC announced the first report of precovery observations for . These precovery images, which were found by Reiner Stoss, came from NEAT observations taken as early as 5 August 2000 (more than two years before the object's official discovery). These remain as the earliest known precoveries of .
Occultations In the sky, may pass in front of a star and block out its light from Earth, producing a stellar occultation. Stellar occultations allow astronomers to accurately determine the size, shape, and position of . Although stellar occultations by were first predicted in 2013, they were not successfully observed until after 2018. The first successful observation of an occultation by took place on 28 January 2018, when it was detected by 12 telescopes across Europe. This occultation provided the first accurate determination of the object's size and shape. Another occultation by took place on 11 November 2021 and was detected by 19 telescopes across Europe and the United States, producing measurements in agreement with those from the previous occultation.
Name This object is currently known by its minor planet provisional designation , given by the MPC upon its discovery. The MPC gave it the minor planet number of 84522 on 4 May 2004. , it has not been named. According to naming guidelines by the International Astronomical Union's Working Group for Small Bodies Nomenclature, trans-Neptunian objects should be given mythological names.
Orbit is a trans-Neptunian object (TNO) located in the scattered disk, a region of the Solar System beyond the orbits of Neptune and Pluto. It follows a highly tilted and elliptical orbit around the Sun (eccentricity 0.29, inclination 35°) at a distance ranging from 39.0 to 71.5 astronomical units (AU). With a semi-major axis or average distance of , takes 411 years to complete one orbit around the Sun. The orbital period of is exactly (or 2.5) times Neptune's orbital period, which means it is in a 2:5 mean-motion resonance with Neptune: for every two revolutions makes around the Sun, Neptune makes exactly five. The 2:5 resonance with Neptune is one of the most common types of orbital resonances seen in the TNO population.
Due to the orbital resonance, Neptune's gravity strongly affects the orbit of , particularly in terms of its eccentricity. Simulations of 's orbit show that in 10 million years, its semi-major axis can fluctuate between , eccentricity between 0.29–0.45, and inclination between 29.2–35.0°. Despite this variability, has a high probability of remaining locked in its resonance, remaining in a stable orbit (without being ejected or falling into the inner Solar System) for at least 5 billion yearslonger than the lifetime of the Solar System itself. , is located above the ecliptic at a distance of from the Sun. It will come to perihelion, its closest distance to the Sun, in October 2058.
Physical characteristics Size and shape
+ History of diameter estimates for Year ofPublicationDiameter(km)MethodReferences2004thermal2005thermal2008thermal2013thermal2020occultation2022occultation
has a mean diameter of approximately , about one-fifth the diameter of Pluto. It is considered a medium-sized or mid-sized TNO, since its diameter lies halfway between those of small, irregularly-shaped bodies and dwarf planets. Stellar occultation observations from 2018 have shown that the shape of likely resembles an oblate spheroid, with a major diameter of and a minor diameter of . A preliminary analysis of another occultation from 2021 found a similar major diameter of and a minor diameter of .
is theoretically massive enough to gravitationally pull itself into a spherical or ellipsoidal shape, which would result in hydrostatic equilibrium. Since this is one of the requirements for becoming a dwarf planet, some astronomers including Michael E. Brown and Noemi Pinilla-Alonso have considered it a possible dwarf planet. However, occultation observations have not ruled out irregularities in 's shape, and its unknown density makes it unclear whether its observed oblate shape is consistent with hydrostatic equilibrium.
was previously considered one of the largest known TNOs during the 2000s. Early measurements of 's thermal emission suggested that its diameter could be as large as , which would have made it half the size of Pluto. After 's diameter was revised downward in 2020, it is now recognized that the initial overestimate of 's diameter was likely caused an unseen moon making it appear larger and brighter than it actually is.
Mass and density The mass and density of have not been measured. These properties could be determined if it has a moon with a known orbital period and distance. In spite of this, astronomers have inferred 's density from its diameter, shape, and rotation period. Based on the known densities of TNOs around 's size, its density is predicted to be around . On the other hand, if is in hydrostatic equilibrium and is rotating rapidly, its density could be at least . Both predicted densities are relatively low for TNOs, which could indicate a porous interior made of ice and rock.
Surface Composition and spectrum
The surface of is very cold, with temperatures reaching as high as at perihelion. Under these conditions, most substances such as water and carbon dioxide remain frozen. Near-infrared spectroscopy by the James Webb Space Telescope (JWST) has identified frozen water, carbon dioxide (), carbon monoxide (CO), methanol (), and large amounts of complex organic compounds (tholins) on the object's surface. This organic-rich composition makes 's surface highly absorbent at near-infrared wavelengths between , producing a "cliff"-shaped reflectance spectrum. For this reason, astronomers classify as an organics-type or "cliff"-type TNO. is richer in methanol compared to other "cliff"-type TNOs, so it is further categorized as a "cliff1"-type or methanol-rich TNO.
A 2026 analysis of 's near-infrared spectrum, led by Lucas McClure and colleagues, suggested that the object's tholins and methanol ice exist in the form of grains between in size. Each of these grains is hypothesized to be embedded with trace amounts (<2% by weight) of amorphous carbon and crystalline water ice, respectively. McClure and colleagues further proposed that 's water ice predominantly exists as amorphous -sized grains containing some embedded ice. On the other hand, a 2023 analysis by Michael E. Brown and Wesley C. Fraser pointed out that absorbs more light than water ice in 's spectrum, and proposed that its water ice is coated by a thin layer of -sized ice particles.
Like many other TNOs, contains more CO ice than expected for its temperature, where CO would ordinarily sublimate and escape. Astronomers hypothesize that the abundant CO in TNOs comes from the irradiation of and methanol through solar and cosmic rays, which break these molecules into CO and other hydrocarbons. The continued irradiation of these resulting hydrocarbons is also thought to produce tholins, which are commonly seen in TNOs. The observed abundances of CO, , and other hydrocarbons on may be related to its distant orbit, which brings it near the termination shock from the Sun and exposes it to higher levels of radiation.
Color and albedo
In visible light, appears dim and red. Astronomers describe its color as "very red" or "ultra red" in comparison to other TNOs. Like all red TNOs, the red coloration of is thought to be caused by tholins on its surface. Very red colors appear to be common among TNOs on highly distant and eccentric orbits like that of , though they are mainly found in the low-inclination ("cold") classical Kuiper belt closer to the Sun.
Based on its brightness and known size, the object's geometric albedo is estimated to be , which is somewhat high compared to other mid-sized TNOs. 's apparently high geometric albedo may be caused by either exposed water ice on its surface, or a hidden moon making it appear brighter than usual. If it has a hidden moon, its true geometric albedo would be around 0.127, closer to the average TNO albedo. The albedo is thought to vary across 's surface because it shows slight variability in brightness as it rotates.
Rotation As it rotates, exhibits slight brightness variability with an amplitude of magnitudes. This small brightness variability makes its rotation period difficult to measure with telescopes. Measurements suggest that rotates slowly, with a period of approximately 56.1 hours. Earlier studies proposed a rotation period of 5.41 hours, but this is now considered unlikely because observational methods tend to favor the detection of shorter periods. The axial tilt of is unknown, as its three-dimensional shape has not been determined through a sufficient number of occultations.
Lack of atmosphere Occultation observations from 2018 have shown that lacks a global atmosphere or a dust coma. The lack of an atmosphere is consistent with theoretical predictions that TNOs smaller than in diameter generally cannot gravitationally hold onto atmospheres over billion-year timescales. If has an undetected atmosphere, the atmospheric pressure at its surface must be less than 100 nanobars.
Possible satellite is not confirmed to have natural satellites or moons, although there is indirect evidence for one orbiting close by. Beginning in 2020, astronomers noticed that emits more thermal radiation than expected for its size. 's excess thermal emission does not fit models of a single, highly emissive object, but could be explained by the presence of a thermally-emitting moon. Based on the difference between 's measured and predicted thermal emission, the diameter of the possible moon is inferred to be (), or about half the diameter of . Such a large moon would make a binary system, likely formed through streaming instability or an impact.