208996 Achlys
Third-largest plutino, elongated by rapid rotation.
208996 Achlys, originally designated 2003 AZ84, is a sizable object in the Kuiper belt, the region of icy bodies beyond Neptune. It was discovered on 13 January 2003 by Chad Trujillo and Michael E. Brown using the 1.22-meter Samuel Oschin telescope at Palomar Observatory, as part of their Caltech Wide Area Sky Survey. The object was identified through manual checks of candidates flagged by automated software, appearing at a red-filter magnitude of 20.2. The Minor Planet Center announced the discovery on 26 January 2003. Later, precovery images were found, including one from 19 March 1996—over six years before the official discovery.
Achlys is a plutino, meaning it follows a 2:3 orbital resonance with Neptune: it completes two orbits for every three of Neptune's. It is the third largest known plutino, after Pluto and Orcus. Its orbit is elliptical and inclined 13.6° to the ecliptic, ranging from 32.4 AU at perihelion to 46.5 AU at aphelion, and it does not cross Neptune's path. It last reached aphelion in June 1982 and will reach perihelion in March 2107. Simulations indicate its perihelion will not drop below 31.6 AU over the next 10 million years, and its orbit is stable for billions of years.
The object is elongated, with an equatorial diameter of about 940 km and a polar diameter of about 490 km. This shape is thought to result from its rapid rotation, which takes 6.8 hours. Its surface is dark gray and composed mainly of water ice. Stellar occultations in 2012 and 2014 revealed a chasm or depression 8 to 13 km deep, similar to features on Pluto and Charon. No rings or additional moons were detected during these events. Achlys has one known moon, roughly 80 km in diameter. Its mass and density have not been measured, but models based on its shape and rotation suggest a density lower than that of water ice.
The object was named after Achlys, the goddess of sorrow and grief in the ancient Greek poem *Shield of Heracles*; in Homer's *Iliad*, "achlys" refers to the mist covering the eyes of the dying. The name follows the IAU convention of underworld-themed names for plutinos and was officially assigned on 30 June 2025. Before that, it was known by its provisional designation 2003 AZ84. Its minor planet number, 208996, was assigned on 11 March 2009.
- discovery_date
- 13 January 2003
- discoverers
- Chad Trujillo and Michael E. Brown
- discovery_site
- Palomar Observatory
- category
- Plutino (Kuiper belt object)
- diameter
- ~940 km (equatorial) × 490 km (polar)
- rotation_period
- ~6.79 hours
- known_moons
- 1 (approx. 80 km diameter)
Lore & Background
Achlys was discovered during the Caltech Wide Area Sky Survey, which used the Palomar Observatory's 1.22-meter Samuel Oschin telescope. The object was found through manual vetting of automated image searches, and its discovery was announced by the Minor Planet Center on 26 January 2003. Precovery observations date back to 19 March 1996, over six years before its official discovery. The object was named after Achlys, the goddess of sorrow and grief from the ancient Greek epic Shield of Heracles, following the IAU's theme of underworld figures for plutinos. Its minor planet catalog number 208996 was assigned on 11 March 2009, and the name was officially approved on 30 June 2025.
Stellar occultations in 2012 and 2014 revealed Achlys's elongated shape, approximated as a Jacobi ellipsoid with dimensions 940 km × 766 km × 490 km. Its surface is dark gray, dominated by water ice (both amorphous and crystalline), with a low geometric albedo of about 10%. A chasm or depression 8–13 km deep was detected on its surface. The object has one known moon about 80 km in diameter. Its mass and density have not been directly measured, but hydrostatic equilibrium models suggest a density of 0.87 g/cm³, lower than water ice, indicating a porous interior.
Reader's Guide
Achlys is significant as the third largest known plutino, after Pluto and Orcus, and as a likely dwarf planet candidate due to its size and probable hydrostatic equilibrium. Its elongated shape, driven by rapid rotation, provides insights into the internal structure and evolution of Kuiper belt objects. The presence of crystalline water ice on its surface suggests past heating events, possibly from impacts. Achlys's stable orbit, which does not cross Neptune's, and its low density contribute to understanding the diversity of trans-Neptunian objects. Its study via stellar occultations has revealed surface topography and shape, while its dark, icy surface and lack of detected rings or additional moons inform models of the outer Solar System's composition and history.
Did You Know?
- Achlys has an elongated shape with an equatorial diameter roughly twice its polar diameter.
- Its surface bears a chasm or depression between 8 and 13 km deep, similar to features on Pluto and Charon.
- Achlys completes one orbit every 247 years, in a 2:3 resonance with Neptune.
- The object's density is predicted to be lower than that of water ice, suggesting a porous interior.
More in Discoveries by Michael E. Brown 1-24
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