208996 Achlys

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Wikipedia

208996 Achlys
208996 Achlys and its possible satellite, imaged by the Hubble Space Telescope on 2 December 2005.
Discovery[1][2]
Discovered byC. Trujillo
M. E. Brown
Discovery siteNEAT–Palomar Obs.
Discovery date13 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 arc20.96 yr (7,654 days)
Earliest precovery date19 March 1996
Aphelion46.555 AU
Perihelion32.170 AU
39.362 AU
Eccentricity0.183
246.96 yr (90,202 days)
232.611°
0° 0m 14.368s / day
Inclination13.596°
252.202°
≈ 27 March 2107[5]
±2.2 days
15.211°
Known satellites1[6] (diameter: 72 km)[7][8]
(unrecovered)[9]
Physical characteristics
Dimensions(940±40) × (766±20) × (490±16) km (derived from the unlikely assumption of hydrostatic equilibrium)[10]
772±12 km (assuming HE)[10]
Mass210×1018 kg (derived from [10])
Mean density
0.87±0.01 g/cm3 (assuming HE)[10]
6.7874±0.0002 h[11]
0.097±0.009 (assuming HE)[10]
20.3 (opposition)[12]
3.760±0.058 (V)[11]
3.537±0.053 (R)[13]

    208996 Achlys (provisional designation 2003 AZ84) is a trans-Neptunian object with a possible moon[6][9] located in the outer regions of the Solar System. It has a mean diameter of over 700 kilometers.[10] 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][14] 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.[15]

    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.[16]: 16  In Homer's Iliad, "achlys" refers to the mist that covers the eyes of the dying.[16]: 16 

    Physical characteristics

    The Spitzer Space Telescope has estimated the diameter of Achlys at 686±96 km,[17] while an analysis of a combination of Spitzer and Herschel data yielded a slightly higher estimate of 727.0+61.9
    −66.5     km    .[18] 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.[9]

    A stellar occultation in 2010 measured a single chord of 573±21 km.[20] 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.[10] 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.[21]

    Orbit and rotation

    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.[14] Since it is in a 2:3 resonance with Neptune, Achlys is classified as a plutino.[14] 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, Achlys is currently located 44.43 AU (6.647×109 km) from the Sun.[12] It had approached its aphelion (furthest distance from the Sun) in 1982[22] 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).[23] 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.[24]

    Polar view of Achlys's orbit (yellow) along with other plutinos.
    Orbit of Achlys (blue) compared to the orbits of Pluto and Neptune.

    Satellite

    Satellite
    Discovery
    Discovered byBrown et al.
    Discovery date2005
    Orbital characteristics[8]
    7200±300 km
    12 d (prograde)
    Satellite ofAchlys
    Physical characteristics[8]
    36±6 km

      Using observations with the Hubble Space Telescope, the discovery of a satellite of Achlys was reported in IAUC 8812 on 22 February 2007.[8][6][25] The object was measured with a separation of 0.22 arcsec and an apparent magnitude difference of 5.0.[6] As of 2012, attempts to recover the satellite have failed.[9] The unrecovered satellite is estimated to be about 72±12 km in diameter.[7][8]

      Notes

      1. ↑ 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.[19] 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.[18]

      References

      1. 1 2 3 4 5 6 7 "JPL Small-Body Database Browser: 208996 Achlys (2003 AZ84)" (2016-03-03 last obs.). Jet Propulsion Laboratory. Retrieved 9 February 2017.
      2. ↑ Marsden, Brian G. (26 January 2003). "MPEC 2003-B27 : 2003 AZ84". Minor Planet Center. Retrieved 6 January 2010.
      3. 1 2 3 Marc W. Buie (18 February 2009). "Orbit Fit and Astrometric record for 208996". Southwest Research Institute. Retrieved 29 August 2009.
      4. 1 2 "(208996) Achlys = 2003 AZ84". Minor Planet Center. Retrieved 9 February 2017.
      5. ↑ JPL Horizons Observer Location: @sun (Perihelion occurs when deldot changes from negative to positive. Uncertainty in time of perihelion is 3-sigma.)
      6. 1 2 3 4 Green, Daniel W. E. (22 February 2007). "Satellites of 2003 AZ_84, (50000), (55637), and (90482)". IAU Circular. 8812: 1. Bibcode:2007IAUC.8812....1B. Archived from the original on 19 July 2011.
      7. 1 2 Johnston, Wm. Robert (24 May 2019). "List of Known Trans-Neptunian Objects". Johnston's Archive. Retrieved 6 July 2019.
      8. 1 2 3 4 5 Johnston, Wm. Robert (20 September 2014). "(208996) 2003 AZ84". Johnston's Archive. Retrieved 16 January 2018.
      9. 1 2 3 4 Brown, Michael E. (3 February 2012). "After a bit more than 3 hrs on 2003AZ84 still no obvious moon". Twitter. Retrieved 3 February 2012.
      10. 1 2 3 4 5 6 7 Dias-Oliveira, A.; Sicardy, B.; Ortiz, J. L.; Braga-Ribas, F.; Leiva, R.; Vieira-Martins, R.; et al. (23 June 2017). "Study of the Plutino Object (208996) 2003 AZ84 from Stellar Occultations: Size, Shape, and Topographic Features". The Astronomical Journal. 154 (1): 13. arXiv:1705.10895. Bibcode:2017AJ....154...22D. doi:10.3847/1538-3881/aa74e9. S2CID 119098862.
      11. 1 2 Santos-Sanz, P.; Lellouch, E.; Groussin, O.; Lacerda, P.; Muller, T. G.; Ortiz, J. L.; Kiss, C.; Vilenius, E.; Stansberry, J.; Duffard, R.; Fornasier, S.; Jorda, L.; Thirouin, A. (August 2017). ""TNOs are Cool": A survey of the trans-Neptunian region XII. Thermal light curves of Haumea, 2003 VS2 and 2003 AZ84 with Herschel/PACS". Astronomy & Astrophysics. 604 (A95): 19. arXiv:1705.09117. Bibcode:2017A&A...604A..95S. doi:10.1051/0004-6361/201630354. S2CID 119489622.
      12. 1 2 "AstDys (208996) 2003AZ84 Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 6 July 2019.
      13. ↑ Peixinho, N.; Delsanti, A.; Guilbert-Lepoutre, A.; Gafeira, R.; Lacerda, P. (9 October 2012). "The bimodal colors of Centaurs and small Kuiper belt objects" (PDF). Astronomy & Astrophysics. 546 (A86): 12. arXiv:1206.3153. Bibcode:2012A&A...546A..86P. doi:10.1051/0004-6361/201219057. S2CID 55876118.
      14. 1 2 3 Marsden, Brian G. (7 August 2009). "MPEC 2009-P26 : Distant Minor Planets (2009 August 17.0 TT)". Minor Planet Center. Retrieved 28 August 2009.
      15. ↑ Grundy, W.M.; Noll, K.S.; Buie, M.W.; Benecchi, S.D.; Ragozzine, D.; Roe, H.G. (7 January 2019), "The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁ'hòmdímà (229762 2007 UK126)" (PDF), Icarus, 334: 30–38, Bibcode:2019Icar..334...30G, doi:10.1016/j.icarus.2018.12.037, S2CID 126574999, archived from the original (PDF) on 7 April 2019, retrieved 7 July 2019
      16. 1 2 "WGSBN Bulletin 5, #15" (PDF). WGSBN Bulletin. 5 (15). International Astronomical Union: 16. 30 June 2025. Retrieved 30 June 2025.
      17. ↑ Stansberry, John; Grundy, Will; Brown, Mike; Cruikshank, Dale; Spencer, John; Trilling, David; et al. (2008). "Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope" (PDF). In Barucci, M. Antonietta; Boehnhardt, Hermann; Cruikshank, Dale P. (eds.). The Solar System Beyond Neptune. University of Arizona Press. pp. 161–179. arXiv:astro-ph/0702538. Bibcode:2008ssbn.book..161S. ISBN 978-0-8165-2755-7.
      18. 1 2 Mommert, M.; Harris, A. W.; Kiss, C.; Pál, A.; Santos-Sanz, P.; Stansberry, J.; et al. (4 May 2012). "TNOs are cool: A survey of the trans-Neptunian region. V. Physical characterization of 18 Plutinos using Herschel-PACS observations". Astronomy & Astrophysics. 541 (A93): 17. arXiv:1202.3657. Bibcode:2012A&A...541A..93M. doi:10.1051/0004-6361/201118562. S2CID 119253817.
      19. ↑ Müller, T. G.; Lellouch, E.; Stansberry, J.; Kiss, C.; Santos-Sanz, P.; Vilenius, E. (16 July 2010). ""TNOs are Cool": A survey of the trans-Neptunian region I. Results from the Herschel science demonstration phase (SDP)". Astronomy & Astrophysics. 518 (L146): L146. arXiv:1005.2923. Bibcode:2010A&A...518L.146M. doi:10.1051/0004-6361/201014683. S2CID 118635387.
      20. ↑ Braga-Ribas, F.; Sicardy, B.; Ortiz, J. L.; Jehin, E.; Camargo, J. I. B.; Assafin, M. (October 2011). Stellar Occultations by TNOs: the January 08, 2011 by (208996) 2003 AZ84 and the May 04, 2011 by (50000) Quaoar (PDF). European Planetary Science Conference 2011. Vol. 6. Bibcode:2011epsc.conf.1060B.
      21. ↑ Delsanti, A.; Merlin, F.; Guilbert-Lepoutre, A.; Bauer, J.; Yang, B.; Meech, K. J. (28 September 2010). "Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of Plutino (90482) Orcus". Astronomy & Astrophysics. 520 (A40): 15. arXiv:1006.4962. Bibcode:2010A&A...520A..40D. doi:10.1051/0004-6361/201014296. S2CID 118745903.
      22. ↑ "Horizon Online Ephemeris System". Jet Propulsion Laboratory. Retrieved 6 July 2019.
      23. ↑ "LCDB Data for (208996)". Asteroid Lightcurve Database (LCDB). Retrieved 9 February 2017.
      24. ↑ Sheppard, Scott S.; Jewitt, David C. (June 2003). "Hawaii Kuiper Belt Variability Project: An Update" (PDF). Earth, Moon, and Planets. 92 (1–4): 207–219. Bibcode:2003EM&P...92..207S. doi:10.1023/B:MOON.0000031943.12968.46. S2CID 189905992.
      25. ↑ Parker, Joel Wm. (15 March 2007). "Distant EKOs #51". The Kuiper Belt Electronic Newsletter. Southwest Research Institute.