120347 Salacia
Possible dwarf planet
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120347 Salacia (provisional designation 2004 SB60) is a large trans-Neptunian object (TNO) and a possible dwarf planet in the Kuiper belt. Its diameter is estimated to be between 875 km (544 mi) and 800 km (500 mi). Salacia orbits the Sun at an average distance that is slightly greater than that of Pluto. It was named after the Roman goddess Salacia and has one single known moon, Actaea. Salacia and Actaea form a binary system where both bodies are tidally locked to the other, similar to the Pluto and Charon system.[8]
Keck Telescope image of Salacia (center) and its moon Actaea (left), taken by the NIRC2 near-infrared camera on 3 August 2010. | |
| Discovery[1][2] | |
|---|---|
| Discovered by | H. G. Roe M. E. Brown K. M. Barkume |
| Discovery site | Palomar Obs. |
| Discovery date | 22 September 2004 |
| Designations | |
| (120347) Salacia | |
| Pronunciation | /sÉËleɪÊÉ/ (sÉ-LAY-shÉ) |
Named after | Salacia (Roman mythology)[2] |
| 2004 SB60 (provisional designation) | |
| TNO[1] · classical (hot)[3] extended[4] | |
| Adjectives | Salacian |
| Symbol | .svg){kind=small} or .svg){kind=small} (rare) |
| Orbital characteristics[1] | |
| Epoch 31 May 2020 (JD 2459000.5) | |
| Uncertainty parameter 3 | |
| Observation arc | 37.16 yr (13,572 days) |
| Earliest precovery date | 25 July 1982 |
| Aphelion | 46.670 AU |
| Perihelion | 37.697 AU |
| 42.184 AU | |
| Eccentricity | 0.10636 |
| 273.98 yr (100,073 days) | |
| 123.138° | |
| 0° 0m 12.951s / day | |
| Inclination | 23.921° |
| 279.880° | |
| 312.294° | |
| Known satellites | 1 (Actaea) |
| Physical characteristics | |
| Mass | 4.861+0.076 â0.074Ã1020 kg (System mass)[8] |
Mean density | 1.50±0.12 g/cm3[6] 1.26±0.16 g/cm3[7] |
| 5.49403±0.00016 d (synchronous)[8] | |
| 0.041±0.004 (2025)[5] 0.042±0.004 (2017)[7] | |
| 20.7 | |
| 4.360±0.011 (Salacia+Actaea)[3] 4.476±0.013 (Salacia)[3] 4.15[1] | |
History
Discovery
Salacia was discovered on 22 September 2004 by American astronomers Henry Roe, Michael Brown and Kristina Barkume at the Palomar Observatory in California, United States.[2]
Name
This minor planet was named after Salacia (/sÉËleɪÊÉ/), the goddess of salt water and the wife of Neptune.[2] The naming citation was published on 18 February 2011 (M.P.C. 73984).[11]
The moon's name, Actaea /ækËtiËÉ/, was assigned on the same date. Actaea is a nereid or sea nymph.
Planetary symbols are no longer used much in astronomy, so Salacia never received a symbol in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a stylised hippocamp (.svg){kind=small}
, formerly .svg){kind=small}
) as the symbol for Salacia;[12][13] although this symbol is not widely used.
Orbit and classification
Orbital characteristics
Salacia orbits the Sun at a distance of 37.7â46.7 AU with a semi-major axis of 42.2 AU, finishing its orbit once every 274.0 years.[1] Salacia is a non-resonant object with a moderate eccentricity of approximately 0.11 and a large inclination of 23.9° with respect to the ecliptic.[1]
Classification
Classical Kuiper belt object
Salacia is located in the classical region of the Kuiper belt (approximately 39â48 AU) from the Sun, therefore, it is classified as a typical member of a classical Kuiper belt object (also known as a "cubewano").[3] The high orbital inclination of Salacia also makes it a dynamically "hot" member of the classical Kuiper belt.[3] The hot classical Kuiper belt objects are believed to have been scattered by Neptune's gravitational influence when the solar system is still young.[14]:â230â
Scatteredâextended object
Gladman et al. suggested that the large inclination of Salacia's orbit makes it a scatteredâextended object in the classification of the Deep Ecliptic Survey but a hot classical Kuiper belt object in the classification system of Gladman et al.,[15] which might be from the same population that formed during the outward migration of Neptune.[3] Salacia's orbit is within the parameter space of the Haumea collisional family, however; Salacia is not part of it, as evidenced by its lack of the strong water-ice absorption bands.[3]
Physical characteristics
In 2019, the total mass of the SalaciaâActaea system was estimated to be at 4.861+0.076
â0.074Ã1020 kg,[8] with an average system density of 1.5±0.1 g/cm3; Salacia itself was estimated to be around 838 km in diameter.[5] Salacia has the lowest albedo of any known large trans-Neptunian object.[3] According to the estimate from 2017 based on an improved thermophysical modelling, the size of Salacia is slightly larger at around 866 km and its density therefore is slightly lower (calculated at 1.26 g/cm3 with the old mass estimate discussed below).[7]
William Grundy et al. describe Salacia as a 'dwarf planetâsized TNO'.[6] They had earlier rejected it as a dwarf planet due to its low estimated density,[16] but backed off that position when they later found out it have a relatively high density of 1.5 g/cm3. Salacia's extremely low albedo of 4% remains consistent with a lack of geological activity that is thought to be typical for many dwarf planets.[6]
Salacia's infrared spectrum is almost featureless, indicating an abundance of water ice of less than 5% on the surface.[17][18] Near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 revealed the presence of water ice on Salacia's surface.[19] No signs of volatile ices such as methane were detected in JWST's spectrum of Salacia.[19] Its light-curve amplitude is estimated at only 3%.[3]
Satellite
â29 km away from Salacia.[8]
Salacia has one known natural satellite, Actaea, that orbits its primary every 5.49380±0.00016 d at a distance of 5700+30
â29 km[8] and with an eccentricity of 0.0084±0.0076. It was discovered on 21 July 2006 by Keith Noll, Harold Levison, Denise Stephens and William Grundy with the Hubble Space Telescope.[20]
Actaea is probably between 360 km (220 mi) and 425 km (264 mi) in diameter.[5] That is approximately one-half the diameter of Salacia; thus, Salacia and Actaea are viewed by William Grundy et al. as a binary system. Actaea has an extremely low albedo of just 0.021±0.004, which is approximately half that of Salacia (0.041±0.004).[5]
It has been calculated that the Salacia system should have undergone enough tidal evolution to circularize their orbits, which is consistent with the low measured eccentricity.[3] The ratio of Actaea's semi-major axis to its primary's Hill radius is 0.0023, which is the tightest out of every known trans-Neptunian binary with a calculated orbit.[3] Salacia and Actaea will occult each other the next time in 2067.[3]
See also
- 307261 Máni, a dwarf planet candidate in the Kuiper belt that is similar to Salacia in size and composition
- 174567 Varda and its moon Ilmarë, a system with similar characteristics
- List of Solar System objects by size
Notes
- approximation if Salacia and Actaea were both spherical and had the same albedo