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208996 Achlys
Plutino From Wikipedia, the free encyclopedia
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208996 Achlys (provisional designation 2003 AZ84) is a large trans-Neptunian planetoid with a possible moon[11][6] located in the outer regions of the Solar System. It has a mean diameter of over 700 kilometers.[7] It belongs to the plutinos – a group of minor planets named after its largest member Pluto – as it orbits in a 2:3 resonance with Neptune in the Kuiper belt.[3][12] It is the third-largest known plutino, after Pluto and Orcus. It was discovered on 13 January 2003, by American astronomers Chad Trujillo and Michael Brown during the NEAT survey using the Samuel Oschin telescope at Palomar 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 into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets.[13]
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Name
On 30 June 2025, the object was given the name Achlys, after the goddess of sorrow and grief in the ancient Greek epic poem Shield of Heracles.[14]: 16 In Homer's Iliad, "achlys" refers to the mist that covers the eyes of the dying.[14]: 16
Physical characteristics
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The Spitzer Space Telescope has estimated the diameter of Achlys at 686±96 km,[15] while an analysis of a combination of Spitzer and Herschel data yielded a slightly higher estimate of 727.0+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 a dwarf 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 stellar occultation in 2010 measured a single chord of 573±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 to Haumea and Varuna.[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 of Orcus, 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 the near-infrared, although Achlys has a lower albedo. Both bodies also have a weak absorption band near 2.3 μm, which may be caused by ammonia hydrate or methane ice.[19]
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Orbit and rotation
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Achlys orbits the Sun at an average distance of 39.4 astronomical units (AU) and completes a full orbit in 247 years.[1] It is in a 2:3 orbital resonance with Neptune; 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 a plutino.[12] Its orbit is inclined to the ecliptic by 13.6 degrees.[1] The orbit of Achlys is moderately eccentric, with an orbital 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 its aphelion (furthest distance from the Sun) in 1982[20] and will come to its perihelion (closest distance to the Sun) in 2107.[1] Simulations by the Deep 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]
The rotation period of this minor planet was first measured by Scott Sheppard in 2003. Light curves obtained by Sheppard at the University of Hawaiʻi's 2.2-meter telescope gave an ambiguous rotation period of either 6.71 or 13.42 hours, with a brightness variation of 0.14 magnitudes (U=2).[21] The shorter rotation period refers to the single-peaked solution, expected if the brightness variations resulted from albedo spots. The longer rotation period is for a double-peaked solution, more consistent with an elongated shape that is rotating edge-on.[22]
Satellite
Using observations with the Hubble Space Telescope, the discovery of a satellite of Achlys was reported in IAUC 8812 on 22 February 2007.[23][11][24] The object was measured with a separation of 0.22 arcsec and an apparent magnitude difference of 5.0.[11] As of 2012[update], attempts to recover the satellite have failed.[6] The unrecovered satellite is estimated to be about 72±12 km in diameter.[25][23]
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Notes
- The results of the previous observation of Achlys by Herschel were reported in 2010. They were obtained using chop/nod technique yielding 910±60 km.[17] The difference can be explained by the large light-curve amplitude of Achlys and the fact that in 2010 the radiation from it was measured at one particular time, while 2012 determination was based on the time-averaged data.[16]
References
External links
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