is an unnamed trans-Neptunian object (TNO) in the Kuiper belt, discovered by Palomar Observatory in December 2002. It is a member of the plutinos, a population of TNOs in a 2:3 orbital resonance with Neptune, like the dwarf planet Pluto. has a diameter of around , which makes it about one-seventh the diameter of Earth's moon and about one-fifth the diameter of Pluto. It has a dark gray surface containing high amounts of water ice and some frozen carbon dioxide, but no detectable signs of frozen methane, nitrogen, or carbon monoxide. The mass, density, shape, and rotation of are all unknown, but are expected to resemble those of other similarly-sized TNOs like .
In 2026, astronomers announced the discovery of a thin atmosphere around , making it likely the first TNO after Pluto discovered to have an atmosphere, and defying expectations that it should be too small to hold onto an atmosphere. With a surface pressure of 100–200 nanobars (roughly 5 million to 10 million times thinner than Earth's atmosphere or 50 to 100 times thinner than Pluto's atmosphere), it is predicted that should lose all of its atmosphere in 100 to 1,000 years. The existence of 's atmosphere suggests it was recently replenished, either by a comet impact or outgassing via cryovolcanic eruptions. The composition of the atmosphere is unknown, but it is thought to contain either nitrogen, argon, or methane.
Discovery and observation history
was discovered on 10 December 2002 (Universal Time; UT) by Palomar Observatory in California, United States. At that time, a team of astronomers led by Chad Trujillo and Michael E. Brown were working in Palomar Observatory using its 1.2-m Samuel Oschin telescope, operating jointly with the Near-Earth Asteroid Tracking (NEAT) program. The object's discovery was announced by the Minor Planet Center (MPC) on 5 January 2003. Precovery observations of from Palomar Observatory were later identified in 2008, with the observations dating to 16 October 1990, 12 October 1991, and 28 September 1992.
On 10 January 2024 (UT), passed in front of a magnitude-15.8 star in the constellation Auriga, causing it to briefly dim from Earth for up to 20 seconds. This event, known as a stellar occultation, was predicted and observed in Japan by a team of astronomers led by Ko Arimatsu of the National Astronomical Observatory of Japan. Telescopes were stationed at Kyoto University, Kiso Observatory, Fukushima Prefecture, and Okayama Observatory, but only the first three sites detected the occultation. All detections showed that the star faded gradually (lasting up to 1.5 seconds) rather than abruptly during the occultation, revealing that had an atmosphere. The discovery was announced on 4 May 2026, making likely the first trans-Neptunian object after Pluto discovered to have an atmosphere.
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 612533 on 28 March 2022. , it has not been named. According to naming guidelines by the International Astronomical Union's Working Group for Small Bodies Nomenclature, trans-Neptunian objects classified as plutinos, such as , should be given mythological names related to the underworld. Since received its minor planet number less than 10 years ago, only the discoverer has the privilege of naming it.
In an interview with Reuters on 4 May 2026, Arimatsu remarked that the current designation of is unmemorable and advocated for a name related to Okinawan mythology, such as Amamikyu (). Michael E. Brown, who was involved in Palomar Observatory's discovery of , commented that the object did not get named because it had no known remarkable properties before Arimatsu's discovery of its atmosphere.
Orbit
orbits the Sun's barycenter with a semi-major axis of 39.3 astronomical units (AU), putting it in the Kuiper belt beyond Neptune's orbit. takes 246 years to complete one orbit, which places it in a 2:3 mean-motion resonance with Neptune. For every two revolutions it makes around the Sun, Neptune makes exactly three. It is therefore a member of the plutinos, a large population of trans-Neptunian objects (TNOs) named after its largest member Pluto.
's orbit is elliptical, with an orbital eccentricity of 0.12, and inclined by 13° with respect to the ecliptic. Its distance from the Sun ranges from at perihelion to at aphelion. will come to perihelion in March 2070. Due to gravitational perturbations by the giant planets, the orbital elements of vary over time. Simulations of 's orbit show that in 10 million years, its semi-major axis can fluctuate between , eccentricity between 0.10–0.15, and inclination between 12.5–13.9°.
Physical characteristics Size
Observations of a stellar occultation in 2024 show that has a radius of , or a diameter of , assuming it has a spherical shape. For comparison, is about one-seventh the diameter of Earth's moon and about one-fifth the diameter of Pluto. The mass, bulk density, and shape of are unknown, though they can be inferred from those of other similarly-sized TNOs like . A 2026 study by Arimatsu and colleagues assumed had a bulk density between 1.0 and , similar to those of Huya and Pluto's moon Charon, respectively. The surface gravity of is estimated to be around 100 times weaker than Earth's. Arimatsu and colleagues expect that has a non-spherical shape due to its relatively small diameter of a few hundred kilometres. The rotation period of is unknown and it has not yet been studied via light curves, according to the Asteroid Lightcurve Database.
Some astronomers, including Noemi Pinilla-Alonso and Michael E. Brown, previously suggested that could be theoretically large enough to become spherical and be possibly a dwarf planet, based on a larger diameter estimate from 2012. This outdated estimate had in diameter, based on mid- to far-infrared measurements of its thermal emission by the Herschel Space Observatory and Spitzer Space Telescope.
Surface Albedo, color, and temperature Measurements of 's visible brightness and infrared thermal radiation suggest that its surface is dark, with a very low geometric albedo of . However, this estimate was made in 2012 using a visible-light (V-band) absolute magnitude and thermal emission-derived diameter of . More recent measurements have since found a brighter V-band absolute magnitude of 4.73 and a smaller diameter of , which should indicate a higher albedo for . However, , no peer-reviewed studies have recalculated 's albedo using newer measurements of its absolute magnitude and diameter.
Telescopic observations in visible light show that the surface color of is gray, similar to some centaurs and Kuiper belt objects including and . Due to 's great distance from the Sun, its surface temperature averages around , cold enough to freeze substances like water into solid ice.
Composition and spectrum
Near-infrared spectroscopy by the James Webb Space Telescope (JWST) has shown that the surface of is rich in water ice, in both amorphous and crystalline phases. The abundance of water ice in 's surface is indicated by the presence of absorption bands at wavelengths of 1.5, 2.0, 3.0, and in the object's near-infrared spectrum, alongside a reflectance peak at due to crystalline water ice. In the Kuiper belt, crystalline water ice tends to transform into amorphous water ice due to irradiation by cosmic rays, though both phases are expected to be in equilibrium (i.e. amounts of crystalline and amorphous water ice remain constant). shares its water ice-rich surface composition with other TNOs, including the plutinos and . These TNOs are classified as "prominent water"-type or "Bowl"-type TNOs, because their near-infrared spectra display a significant bowl-shaped water ice absorption band at the wavelength of .
Besides water, JWST has detected frozen carbon dioxide () on 's surface. JWST has also detected trace amounts of 13 (a rare isotopologue of ). In observations from 2022 and 2024, JWST found no signs of hypervolatile compounds such as methane, nitrogen, and carbon monoxide (which can sublimate into gases at 's temperature). The lack of hypervolatiles on 's surface suggests that most of them had already sublimated and escaped into space.
Atmosphere
In May 2026, a research team led by Ko Arimatsu announced the discovery of a thin atmosphere around , making it possibly the first TNO after Pluto discovered to have an atmosphere. The atmosphere was inferred from observations of a stellar occultation in January 2024, when passed in front of a star and dimmed its light gradually (lasting 1.5 seconds) instead of abruptly, indicating refraction by an atmosphere. The gradual disappearance and reappearance of the star was observed to occur within radii of from 's center. Alternate explanations for this gradual fading, such as a ring system or a shell of dust, are considered unlikely.
Based on the observed fading of the occulted star, Arimatsu's team estimated that the atmospheric pressure on 's surface is between 100 and 200 nanobars—roughly 5 million to 10 million times thinner than Earth's atmosphere and about 50 to 100 times thinner than Pluto's atmosphere, but about 100 to 200 times denser than Europa's exosphere. At this pressure, 's atmosphere is dense enough (>0.001 nanobar) that its gas molecules can collide amongst themselves, making it a true atmosphere rather than an exosphere. No other TNO besides Pluto is known to have an atmospheric pressure this high. However, it is unclear how the density of 's atmosphere changes with altitude.
The composition 's atmosphere is unknown, and there are competing hypotheses for it. One possible composition is methane, a common hypervolatile found in TNOs. In occultation observations, 's atmosphere appeared to show a sudden change in refraction at a certain altitude, suggesting that it has a thermal inversion caused by upper-atmospheric heating of trace methane gas (similar to Pluto). However, JWST observed the object after its 2024 occultation and found no signs of methane and carbon monoxide gas, suggesting they do not make up much of its atmosphere. If these gases exist over a large altitude range, their partial pressures at 's surface must be less than 0.0003 nanobars (0.3 picobars). However, it remains possible that a low-altitude atmosphere of pure methane could exist. Using these JWST observations, a research team headed by Ian Wong proposed that 's atmosphere could contain either nitrogen, argon, or any other gas that is unable to be detected by JWST.
The discovery of an atmosphere around was unexpected because the object is too small to have a surface gravity strong enough to theoretically hold on to an atmosphere over the age of the Solar System. Physical models predict that should lose all of its atmosphere within 100 to 1,000 years via Jeans escape. The present-day existence of 's atmosphere implies that it was recently replenished, though the cause is unknown. Sublimation of volatile ices on 's surface is unlikely, because it was not detected in earlier JWST observations. Arimatsu's team hypothesized that obtained its atmosphere via either an impact by a hundred-meter-radius comet, or outgassing by cryovolcanic eruptions. These hypotheses can be tested with later observations by occultations: if the atmosphere came from an impact, then its surface pressure should decrease over time, whereas if the atmosphere is sustained by outgassing, then the surface pressure should stay consistent with possibly slow seasonal changes.
See also 208996 Achlys – a large plutino and possible dwarf planet with an elongated ellipsoid shape , a similarly-sized plutino whose shape has been characterized by stellar occultations , another similarly-sized trans-Neptunian object whose shape has been characterized by stellar occultations