120347 Salacia (provisional designation ) is a large trans-Neptunian object in the Kuiper belt. It was discovered on 22 September 2004 by American astronomers Henry G. Roe, Michael E. Brown, and Kristina Barkume using the Hale Reflector at the Palomar Observatory. Its discovery was officially announced in May 2005. Salacia orbits the Sun at an average distance slightly greater than that of Pluto, and is classified as a hot cubewano or a scattered–extended object. It was named after the Roman sea goddess Salacia.
Salacia is a possible dwarf planet with an estimated diameter of , and a low light curve, implying a nearly spherical shape. While older size estimates suggested a low density and highly porous interior, newer measurements calculate a higher density between and . It has a dark gray surface with a neutral color index and an exceptionally low albedo of . This low albedo indicates a lack of geological activity. Observations by the James Webb Space Telescope (JWST) showed that water ice makes up less than 5% of its surface, detecting both amorphous and crystalline water ice alongside carbon dioxide ice. It lacks volatile super-ices like methane, placing it in the "prominent water" (H2O-type) TNO group alongside objects like 307261 Máni and 90482 Orcus.
Salacia has one known moon, Actaea, and their combined system mass is . The primary body and its moon form a fully tidally locked system with a synchronous rotation period of 5.49 days, similar to the Pluto–Charon system. The two bodies will pass directly in front of each other during a rare mutual occultation season starting in 2067.
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. The discovery observations were formally published in May 2005 in the Minor Planet Electronic Circular (MPEC) 2005-J26. The object was announced alongside a group of seven new trans-Neptunian objects tracked by the IAU's Minor Planet Center.
At the time of its initial detection on 22 September, Salacia was recorded with a red-filter apparent magnitude ranging between 20.1 and 20.6. Following its initial tracking, the object was assigned the provisional designation . While the discovery tracking occurred in 2004, astronomers successfully verified earlier positions of the body through archival precovery images taken at the Siding Spring Observatory (Station 260) dating back to 25 July 1982.
Further observations The observation arc of Salacia begins with a precovery observation on 25 July 1982 at the Siding Spring Observatory (Station 260). Between 2001 and 2003, Salacia was pictured multiple times by the Near-Earth Asteroid Tracking (NEAT) program at Palomar (Station 644) and the Sloan Digital Sky Survey at Apache Point (Station 645). After its discovery in 2004, it was observed by the 5-meter Hale Reflector (Station 675) and the 3.5-meter telescope at Mauna Kea (Station 568). As of May 2026, a total of 490 observations have been recorded over a period of 37.16 years (13,572 days). These observations were made by ground-based and space-based telescopes, such as the Hubble Space Telescope (Station 250). More recent tracking was done by the Pan-STARRS 1 survey (Station F51), the Weizmann Astrophysical Observatory (Station M01), and the Pises Observatory (Station 122). The most recent observation recorded in the Minor Planet Center database was done on 1 November 2025, at F51 – Pan-STARRS 1, Haleakalā.
Nomenclature Numbering and naming
This minor planet was named after Salacia ( , ), the goddess of salt water, consort of Neptune. She is the Roman equivalent of the Greek sea goddess Amphitrite or Tethys. The object had been assigned its permanent minor planet number (120347) on 16 November 2005, following the precise consolidation of its orbital path data. The official naming citation was subsequently published on 18 February 2011 in M.P.C. 73984, which formally established the name within the International Astronomical Union's minor planet index. Its moon's name is called Actaea (; . It was from or 'rocky shore') being one of the 50 Nereid sea-nymphs, matching the system's marine naming theme.Homer, Iliad 18.41; Apollodorus, 1.2.7; Hesiod, Theogony 240
Planetary symbol
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 (, formerly ) as the symbol for Salacia; although this symbol is not widely used.
Orbit and classification
Salacia orbits the Sun at a distance of 37.7–46.7 AU with a semi-major axis of 42.2 AU, finishing its orbit once every 274.0 years. 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. The orbit of Salacia is similar to Pluto's, except for a near opposite longitude of ascending node. Its current position is near its most northern position above the ecliptic.
Salacia is located in the classical region of the Kuiper belt between 39 and 48 AU from the Sun. Because it lacks an active orbital resonance with Neptune, it is grouped as a typical member of the classical Kuiper belt population, commonly called "cubewanos". However, Salacia's steep orbital inclination of 23.9° places it within the dynamically "hot" sub-population of the classical belt. Unlike cold classical objects that formed in place along circular paths, hot classical bodies likely formed closer to the Sun and were scattered outward into highly tilted orbits during the early migration of Neptune.
Astronomers utilize different frameworks to sort these outer Solar System bodies, leading to a long-standing academic debate over Salacia's exact status. In the Deep Ecliptic Survey framework by Elliot et al., Salacia is grouped as a scattered–extended object because the system automatically places highly tilted orbits into the scattered categories. Conversely, Brett Gladman and his team reject this label based on a 10-million-year computer simulation testing orbital stability. Because Salacia's orbit stays perfectly stable and completely clear of Neptune's gravity during the simulation, Gladman et al. formally classify it as a hot classical Kuiper belt object. This orbital path also places it near the Haumea collisional family by coincidence; however, tracking and space-based spectral data confirm Salacia is an unrelated background object despite its water-ice rich surface.
Observability Occultations + Salacia stellar occultationsDateStar apparent magnitude (G-mag)OccultationdurationMeasured chordlengthTarget bodyObserving asset /Network27 Dec 201815.566.7 s (maximum duration)SalaciaLESIA Lucky Star28 Jul 2023SalaciaLa Silla 1.54m Telescope17 Aug 202415.948.2 s (maximum duration)Salacia & ActaeaHubble / LESIA26 Jan 202713.942.5 s (predicted)SalaciaSwRI RECON network
Stellar occultations by Salacia happen when it passes in front of a background star, letting scientists measure its precise position, shape, and size profile. To make predictions more precise, astronomers map Salacia's orbit using wide-field star catalogs, while a 2015 study identified four potential occultations visible by the James Webb Space Telescope using faint background stars between infrared magnitudes 14 and 16. Several recorded campaigns have progressively narrowed Salacia's orbital path from an early uncertainty of about . On 27 December 2018, an event monitored by the LESIA - Observatoire de Paris against a Gaia magnitude 15.5 star captured a 4.8 magnitude drop, significantly refining the orbit from an initial ground-based uncertainty of . Following this, a 28 July 2023 occultation captured by the 1.54m Danish Telescope at La Silla recorded a light drop lasting at a relative speed of , providing a direct single-chord surface size measurement of . A rare double stellar occultation was recorded on 17 August 2024, when both Salacia and its moon, Actaea, blocked the same Gaia magnitude 15.9 star within 5 minutes of each other. Because the initial ground-based path carried a massive uncertainty of , astronomers used the Hubble Space Telescope under Proposal 17848 to take precise photos beforehand, reducing the uncertainty to just . This allowed ground stations to align their telescopes perfectly, recording a sharp 5.0 magnitude drop to measure both bodies, while the RECON network is targeting a future occultation of a magnitude 13.9 star scheduled for 26 January 2027.
Rotation and hypothesized shape Light curves and shape
Salacia has a very low light curve amplitude of only about 3%, meaning its brightness does not change much as it spins. Accurate tracking measures this variation at magnitudes. This small change suggests that Salacia is either nearly a perfect sphere or its pole is pointing almost directly at Earth.
In planetary science, minor light variations can be caused by either an elongated shape or by dark and light spots on a round body. An elongated shape would produce a double-peaked light curve with two minimums and maximums per rotation. Because Salacia's light variations show a single-peaked pattern with only one peak per cycle, astronomers explicitly reject the elongated triaxial ellipsoid shape idea. Instead, the light curve is entirely driven by uneven albedo variations across its surface, so Salacia is unlikely to be an elongated triaxial ellipsoid, but rather having a nearly spherical shape with small albedo variations across its surface.
Rotational speed and mutual tidal locking In 2014, early studies estimated that Salacia rotated very fast, once every 6.09–6.5 hours. However, a long-term study published in 2025 showed this was a mathematical error called "observational aliasing." Because ground telescopes could only watch Salacia for a few days at a time, the gaps in the data made a slow 5.49-day spin look like a fast 6.5-hour cycle.
The 2025 study analyzed 165 hours of data collected over 16 years. By combining ground tracking with images from the Hubble Space Telescope, scientists found that Salacia's 5.49-day rotation exactly matches the time its moon, Actaea, takes to complete one orbit ().
This means the two bodies are mutually tidally locked in a "doubly synchronous rotation," always showing the exact same face to each other just like Pluto and Charon. Before this happened, Salacia likely spun much faster, at around 10 hours per rotation. Because Actaea is relatively large, its gravitational pull created internal friction that gradually slowed Salacia down. Computer models show this tidal slowing took only 150 million to 400 million years to finish, meaning the pair has been locked in this balance for billions of years.
Separating the individual light data also revealed features about the moon itself. While Salacia shows small brightness changes, Actaea exhibits a very high light curve amplitude of at least 0.95 magnitudes. This indicates that Actaea either has a highly stretched shape or extreme surface contrasts with large dark and light patches. This discovery cements the system as only the third confirmed doubly synchronous binary in the trans-Neptunian region, joining Pluto–Charon and Eris–Dysnomia.
Physical characteristics
Size, mass and density
+ History of diameter estimates for Salacia Year of Publication Diameter (km) Method References 2012 thermal (Spitzer) 2013 thermal (Herschel) 2013 thermal (Spitzer) 2017 dynamical (HST) 2019 thermal (combined) 2025 thermal (ALMA/NEATM)