Sedna (minor-planet number 90377) is a dwarf planet in the outermost reaches of the Solar System, orbiting the Sun far beyond the orbit of Neptune. It was discovered in 2003, and is roughly 1,000 km in diameter. Spectroscopic analysis has revealed its surface to be a mixture of the solid ices of water, carbon dioxide, and ethane, along with sedimentary deposits of methane-derived, reddish-colored tholins, a chemical makeup similar to the surfaces of other trans-Neptunian objects. Sedna is not expected to have a substantial atmosphere. Within the range of uncertainty, it is tied with in the asteroid belt as the largest dwarf planet not known to have a moon. Owing to its lack of known moons, Sedna's mass and density remain unknown.
Sedna takes approximately 11,400 years to complete one orbit around the Sun. Its orbit is one of the widest known in the Solar System. Its aphelion is located 937 astronomical units (AU) away, about 19 times farther than that of Pluto. Sedna's orbit is also one of the most elliptical discovered, with an eccentricity of 0.85. , Sedna is from the Sun,NASA JPL Horizons ephemeris 2025–2035 2.5 times as far away as Neptune.
Upon its discovery, Sedna was initially classified as a member of the scattered disc, a group of objects sent into high-eccentricity orbits by the gravitational influence of Neptune. Some astronomers instead referred to it as the first known member of the inner Oort cloud, as its perihelion at is far too distant for it to have been scattered by any of the known planets. It has since become the prototype of a new class of objects characterized by highly eccentric orbits with very distant perihelia, the sednoids. The astronomer Michael E. Brown, co-discoverer of Sedna, has argued that its unusual orbit could provide information on the early evolution of the Solar System. Sedna might have been perturbed into its orbit through a close gravitational encounter with the hypothetical Planet Nine.
History
Discovery Sedna (provisionally designated ) was discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) on 14 November 2003. The discovery formed part of a survey begun in 2001 with the Samuel Oschin telescope at Palomar Observatory near San Diego, California, using Yale's 160-megapixel Palomar Quest camera. On that day, an object was observed to move by 4.6 arcseconds over 3.1 hours relative to stars, which indicated that its distance was about 100 AU. Follow-up observations were made in November–December 2003 with the SMARTS (Small and Medium Research Telescope System) at Cerro Tololo Inter-American Observatory in Chile, the Tenagra IV telescope in Nogales, Arizona, and the Keck Observatory on Mauna Kea in Hawaii. Combined with precovery observations taken at the Samuel Oschin telescope in August 2003, and by the Near-Earth Asteroid Tracking consortium in 2001–2002, these observations allowed the accurate determination of its orbit. The calculations showed that the object was moving along a distant and highly eccentric orbit, at a distance of 90.3 AU from the Sun. Precovery images have since been found in the Palomar Digitized Sky Survey dating back to 25 September 1990.
Naming Brown initially nicknamed Sedna "The Flying Dutchman", or "Dutch", after a legendary ghost ship, because its slow movement had initially masked its presence from his team. He eventually settled on the official name after the goddess Sedna from Inuit mythology, partly because he mistakenly thought the Inuit were the closest polar culture to his home in Pasadena, and partly because the name, unlike Quaoar, would be easily pronounceable by English speakers. Brown further justified his choice of naming by stating that the goddess Sedna's traditional location at the bottom of the Arctic Ocean reflected Sedna's large distance from the Sun. He suggested to the International Astronomical Union's (IAU) Minor Planet Center that any objects discovered in Sedna's orbital region in the future should be named after mythical entities in Arctic mythologies.
The team made the name "Sedna" public before the object had been officially numbered, which caused some controversy among the community of amateur astronomers. Brian Marsden, the head of the Minor Planet Center, stated that such an action was a violation of protocol, and that some members of the IAU might vote against it. One amateur astronomer, Reiner Stoss, unsuccessfully attempted to name one of his asteroid discoveries "Sedna" (after the singer Katy Sedna) in protest of Brown's premature naming. Despite the complaints by amateur astronomers, no objection was raised to Brown's name by members of the IAU and no competing names were suggested for Brown's object. The IAU's Committee on Small Body Nomenclature accepted the name in September 2004, and considered that, in similar cases of extraordinary interest, it might in the future allow names to be announced before they were officially numbered.
Sedna has no symbol in astronomical literature, as the usage of planetary symbols is discouraged in astronomy. Unicode includes a symbol (U+2BF2), but this is mostly used among astrologers. The symbol is a monogram of Sanna, the modern pronunciation of Sedna's name.
Orbit and rotation
Sedna has the longest orbital period of any known object in the Solar System of its size or larger with an orbital period of around 11,400 years. Its orbit is extremely eccentric, with an aphelion of approximately 937 AU and a perihelion of 76.19 AU. Near aphelion, Sedna is one of the coldest places in the Solar System, located far past the termination shock, where temperatures never exceed due to its extreme distance. At aphelion, the Sun as viewed from Sedna is a particularly bright star, among the other stars, in the otherwise black sky, being about 45% as bright as the full moon as seen from Earth.Apparent magnitude of Sun at 937 AU = −26.74 + 5 log(937) = −11.88. Full moon magnitude is −12.74. Ratio is Its perihelion was the largest for any known Solar System object until the discovery of the sednoid . At its aphelion, Sedna orbits the Sun at a meagre 377 m/s,Calculated: https://www.wolframalpha.com/input?i=G*solar+mass%2FAU%281%2F76.19-1%2F937%29%3D1%2F2*%28x+m%2Fs%29%5E2%28%28937%2F76.19%29%5E2-1%29 1.3% that of Earth's average orbital speed.377.4 m/s for Sedna divided by 29.78 km/s for Earth.
When Sedna was first discovered, it was 89.6 AU away from the Sun, approaching perihelion, and was the most distant object in the Solar System observed. Sedna was later surpassed by Eris, which was detected by the same survey near its aphelion at 97 AU. Because Sedna is near perihelion , both Eris and are farther from the Sun, at 96 AU and 89 AU respectively, than Sedna at 84 AU, despite both of their semi-major axes being shorter than Sedna's. The orbits of some long-period comets extend further than that of Sedna; they are too dim to be discovered except when approaching perihelion in the inner Solar System. As Sedna nears its perihelion in mid-2076, the Sun will appear merely as a very bright pinpoint in its sky, too far away to be visible as a disc to the naked eye.
When first discovered, Sedna was thought to have an unusually long rotational period (20 to 50 days). It was initially speculated that Sedna's rotation was slowed by the gravitational pull of a large binary companion, similar to Pluto's moon Charon. However, a search for such a satellite by the Hubble Space Telescope in March 2004 found no such objects. Subsequent measurements from the MMT telescope showed that Sedna in reality has a much shorter rotation period of about 10 hours, more typical for a body its size. It could rotate in about 18 hours instead, but this is thought to be unlikely.
Physical characteristics
In visible light, Sedna has an absolute magnitude of about 1.8 and an estimated albedo (reflectivity) of around 0.41, which gives it a diameter of approximately 900 km. At the time of discovery it was the brightest object found in the Solar System since Pluto in 1930. In 2004, the discoverers placed an upper limit of 1,800 km on its diameter; after observations by the Spitzer Space Telescope, this was revised downward by 2007 to less than 1,600 km. In 2012, measurements from the Herschel Space Observatory suggested that Sedna's diameter was , which would make it smaller than Pluto's moon Charon. In 2013, the same team re-analyzed Sedna's thermal data with an improved thermophysical model and found a consistent value of , suggesting that the original model fit was too precise. Australian observations of a stellar occultation by Sedna in 2013 produced similar results on its diameter, giving chord lengths and . The size of this object suggests it could have undergone differentiation and may have a sub-surface liquid ocean and possibly geologic activity.
As Sedna has no known moons, the direct determination of its mass is as yet impossible without either sending a space probe or perhaps locating a nearby object which is gravitationally perturbed by the dwarf planet. It is the largest trans-Neptunian Sun-orbiting object not known to have a natural satellite. As of 2024, observations from the Hubble Space Telescope in 2004 have been the only published attempt to find a satellite, and it is possible that a satellite could have been lost in the glare from Sedna itself.
Observations from the SMARTS telescope show that Sedna, in visible light, is one of the reddest objects known in the Solar System, nearly as red as Mars. Its deep red spectral slope is indicative of high concentrations of organic material on its surface. Chad Trujillo and his colleagues suggest that Sedna's dark red color is caused by an extensive surface coating of hydrocarbon sludge, termed tholins. Tholins are a reddish-colored, amorphous, and heterogeneous organic mixture hypothesized to have been transmuted from simpler organic compounds, following billions of years of continuous exposure to ultraviolet radiation, interstellar particles, and other harsh environs as the dwarf planet either comes close to the Sun or transits interstellar space. Its surface is homogeneous in color and spectrum; this may be because Sedna, unlike objects nearer the Sun, is rarely impacted by other bodies, which would expose bright patches of fresh icy material like that on 8405 Asbolus. Sedna and two other very distant objects – and – share their color with outer classical Kuiper belt objects and the centaur 5145 Pholus, suggesting a similar region of origin.
Trujillo and colleagues have placed upper limits on Sedna's surface composition of 60% for methane ice and 70% for water ice. The presence of methane further supports the existence of tholins on Sedna's surface, as methane is among the organic compounds capable of giving rise to tholins. Barucci and colleagues compared Sedna's spectrum with that of Triton and detected weak absorption bands belonging to methane and nitrogen ices. From these observations, they suggested the following model of the surface: 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen ices, 26% methanol, and 33% methane. The detection of methane and water ice was confirmed in 2006 by the Spitzer Space Telescope mid-infrared photometry. The European Southern Observatory's Very Large Telescope observed Sedna with the SINFONI near-infrared spectrometer, finding indications of tholins and water ice on the surface.
In 2022, low-resolution near-infrared (0.7–5 μm) spectroscopic observations by the James Webb Space Telescope (JWST) revealed the presence of significant amounts of ethane ice (C2H6) and of complex organics on the surface of Sedna. The JWST spectra also contain evidence of the existence of small amounts of ethylene (C2H4), acetylene (C2H2) and possibly carbon dioxide (CO2). On the other hand little evidence of the existence of methane (CH4) and nitrogen ices was found at variance with the earlier observations.