 Achlys and its moon (upper right) imaged by theHubble Space Telescope in December 2005 | |
| Discovery[1][2] | |
|---|---|
| Discovered by | C. Trujillo M. E. Brown |
| Discovery site | NEAT–Palomar Obs. |
| Discovery date | 13 January 2003 |
| Designations | |
| (208996) Achlys | |
| Pronunciation | /ˈækləs/ |
Named after | Achlys |
| 2003 AZ84 | |
| TNO[1] · plutino[3] · distant[4] | |
| Orbital characteristics[1] | |
| Epoch 27 April 2019 (JD 2458600.5) | |
| Uncertainty parameter 3 | |
| Observation arc | 20.96 yr (7,654 days) |
| Earliestprecovery date | 19 March 1996 |
| Aphelion | 46.555AU |
| Perihelion | 32.170 AU |
| 39.362 AU | |
| Eccentricity | 0.183 |
| 246.96yr (90,202 days) | |
| 232.611° | |
| 0° 0m 14.368s / day | |
| Inclination | 13.596° |
| 252.202° | |
| ≈ 27 March 2107[5] ±2.2 days | |
| 15.211° | |
| Knownsatellites | 1 (unrecovered)[6] |
| Physical characteristics | |
| Dimensions | (940±40) × (766±20) × (490±16) km(derived from the unlikely assumption ofhydrostatic equilibrium)[7] |
| 772±12 km(area equivalent, assuming HE)[7] | |
| Mass | 210×1018 kg(derived from[7]) |
Meandensity | 0.87±0.01 g/cm3(assuming HE)[7] |
| 6.7874±0.0002h[8] | |
| 0.097±0.009(assuming HE)[7] | |
| 20.3(opposition)[9] | |
| 3.760±0.058(V)[8] 3.537±0.053(R)[10] | |
208996 Achlys (provisional designation2003 AZ84) is a largetrans-Neptunian object with a possiblemoon[11][6] located in the outer regions of theSolar System. It has a mean diameter of over 700 kilometers.[7] It belongs to theplutinos – a group ofKuiper belt objects named after its largest memberPluto – as it orbits in a2:3 resonance with Neptune in theKuiper belt.[3][12] It is the third-largest known plutino, afterPluto andOrcus. It was discovered on 13 January 2003, by American astronomersChad Trujillo andMichael Brown during theNEAT survey using theSamuel Oschin telescope atPalomar Observatory.[4]
The low density of this and many other mid-sized TNOs implies that they have never compressed into fully solid bodies, let alone differentiated or collapsed intohydrostatic equilibrium, and so are highly unlikely to be dwarf planets.[13]
On 30 June 2025, the object was given the nameAchlys, after the goddess of sorrow and grief in theancient Greekepic poemShield of Heracles.[14]: 16 InHomer'sIliad, "achlys" refers to the mist that covers the eyes of the dying.[14]: 16
TheSpitzer Space Telescope has estimated the diameter of Achlys at686±96 km,[15] while an analysis of a combination of Spitzer andHerschel data yielded a slightly higher estimate of727.0+61.9
−66.5 km.[16] These results are in agreement with each other.[a]The large size of Achlys suggests the possibility of it being adwarf planet. However, if one assumes it to be in hydrostatic equilibrium, as it would be if it were a dwarf planet, the density that results is too low for it to be solid, and hence it could not be a dwarf planet. However, its density cannot be directly calculated because its mass is unknown, due to its satellite not having been recovered.[6]
A stellaroccultation in 2010 measured a singlechord of573±21 km.[18] In 2017, new stellar occultation data suggested that Achlys either had topographic features of up to 40 km above its limb, or had a markedly elongated shape, presumably due to a rapid rotation rate of 6.71 hours calculated from its rotational lightcurve. Such a shape would be similar toHaumea andVaruna.[7] Assuming Achlys is in hydrostatic equilibrium, that would mean dimensions of approximately 940×766×490 km, with its longest axis nearly twice as long as its polar axis.
The spectra and colors of Achlys are very similar to those ofOrcus, another large object in 2:3 resonance with Neptune. Both bodies have a flat featureless spectrum in the visible and moderately strong water ice absorption bands in thenear-infrared, although Achlys has a lower albedo. Both bodies also have a weak absorption band near 2.3 μm, which may be caused byammonia hydrate ormethane ice.[19]
Achlys orbits theSun at an average distance of 39.4 astronomical units (AU) and completes a full orbit in 247 years.[1] It is in a 2:3orbital resonance withNeptune; Achlys completes two orbits around the Sun for every three orbits completed by Neptune.[12] Since it is in a 2:3 resonance with Neptune, Achlys is classified as aplutino.[12] Its orbit isinclined to theecliptic by 13.6 degrees.[1] The orbit of Achlys is moderately eccentric, with anorbital eccentricity of 0.183.[1] As of July 2019[update], Achlys is currently located 44.43 AU (6.647×109 km) from the Sun.[9] It had approached itsaphelion (furthest distance from the Sun) in 1982[20] and will come to itsperihelion (closest distance to the Sun) in 2107.[1] Simulations by theDeep Ecliptic Survey show that over the next 10 million years Achlys will not come closer (qmin) than 31.6 AU from the Sun (it will stay farther away than Neptune).[3]
Therotation period of this minor planet was first measured byScott Sheppard in 2003.Light curves obtained by Sheppard at theUniversity of Hawaiʻi's2.2-meter telescope gave an ambiguous rotation period of either 6.71 or 13.42 hours, with a brightness variation of 0.14magnitudes (U=2).[21] The shorter rotation period refers to the single-peaked solution, expected if the brightness variations resulted fromalbedo spots. The longer rotation period is for a double-peaked solution, more consistent with an elongated shape that is rotating edge-on.[22]
| Discovery | |
|---|---|
| Discovered by | Michael E. Brown Terry-Ann Suer |
| Discovery date | 2 December 2005[11] |
| Orbital characteristics[23] | |
| 7200±300 km[23] or ~10000 km[7]: 2 | |
| 12 d (calculated)[23] | |
| Satellite of | Achlys |
| Physical characteristics | |
| 72±12 km (calculated)[23] or ~80 km (same albedo as Achlys)[7]: 2 | |
Achlys has one knownnatural satellite or moon, which has no official name or designation.[1][23] It was discovered by Michael E. Brown and Terry-Ann Suer in images taken by theHubble Space Telescope on 2 December 2005. The discovery of Achlys's moon alongside the moons ofQuaoar,Orcus, andUni were announced on 22 February 2007 via anInternational Astronomical Union Circular published by theCentral Bureau for Astronomical Telegrams.[23][11]
In the discovery images, the moon was seen at anangular separation of0.22±0.01arcseconds from Achlys,[11] which translates to an apparent distance of at least 7,200 ± 300 km (4,470 ± 190 mi).[23][b] The moon is5.0±0.3magnitudes fainter than Achlys, which translates to a diameter of roughly 80 km (50 mi) if it has the same albedo as Achlys.[23][7]: 2
Although the Hubble Space Telescope reobserved Achlys in 2007, it did not redetect its moon.[24] In February 2012, Brown reported onTwitter that he was unable to find Achlys's moon in images taken by theKeck Telescope.[6] The lack of redetections means that the orbit of Achlys's moon could not be determined, which prevents an accurate determination of Achlys's mass.[7]: 2
| Consensus | _(cropped).jpg)  | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Candidate (for TNOs, D+1σ ≥ 700 km or H ≤ 4.0) |
| ||||||||||||
| Planetary satellites of |    | |
|---|---|---|
| Dwarf planet satellites of | ||
| Minor-planet moons | ||
| Ranked by size | ||
