Sedna (dwarf planet)

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. 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. 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. 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. 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. 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. Its perihelion was the largest for any known Solar System object until the discovery of the sednoid . 1.3% that of Earth's average orbital speed. 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 ==

's Sloan Digital Sky Survey on 11 and 12 October 2005, showing the object's slow movement across the sky 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. 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. 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. The possible presence of nitrogen on the surface suggests that, at least for a short time, Sedna may have a tenuous atmosphere. During the 200-year portion of its orbit near perihelion, the maximum temperature on Sedna should exceed , the transition temperature between alpha-phase solid N2 and the beta-phase seen on Triton. At 38 K, the N2 vapor pressure would be 14 microbar (1.4 Pa). The weak methane absorption bands indicate that methane on Sedna's surface is ancient, as opposed to being freshly deposited. This finding indicates that Sedna's surface never reaches a temperature high enough for methane on the surface to evaporate and subsequently fall back as snow, which happens on Triton and probably on Pluto. == Origin ==

In their paper announcing the discovery of Sedna, Brown and his colleagues described it as the first observed body belonging to the Oort cloud, the hypothetical cloud of comet-like objects thought to exist out to nearly a light-year from the Sun. They observed that, unlike scattered disc objects such as Eris, Sedna's perihelion (76 AU) is too distant for it to have been scattered by the gravitational influence of Neptune. This planet would be perhaps six times as massive as Earth. It would have a highly eccentric orbit, and its average distance from the Sun would be about 15 times that of Neptune (which orbits at an average distance of ). Accordingly, its orbital period would be approximately 7,000 to 15,000 years. or a main-sequence star 80 percent more massive than the Sun, which, owing to its larger mass, may now be a white dwarf. In either case, the stellar encounter had likely occurred within 100 million years after the Sun's formation. Stellar encounters during this time would have minimal effect on the Oort cloud's final mass and population since the Sun had excess material for replenishing the Oort cloud. == Population ==

() with 100 AU grids for scale|alt=Three overlapping ovals represent the orbits Sedna's highly elliptical orbit, and thus a narrow temporal window for detection and observation with currently available technology, means that the probability of its detection was roughly 1 in 80. Unless its discovery were a fluke, it is expected that another 40–120 Sedna-sized objects with roughly the same orbital parameters would exist in the outer solar system. == Classification ==

The discovery of Sedna renewed the old question of just which astronomical objects ought to be considered planets, and which ones ought not to be. On 15 March 2004, articles on Sedna in the popular press reported that a tenth planet had been discovered. This question was resolved for many astronomers by applying the International Astronomical Union's definition of a planet, adopted on 24 August 2006, which mandated that a planet must have cleared the neighborhood around its orbit. Sedna is not expected to have cleared its neighborhood; quantitatively speaking, its Stern–Levison parameter is estimated to be much less than 1. The IAU also adopted dwarf planet as a term for the largest non-planets (despite the name) that, like planets, are in hydrostatic equilibrium and thus can display planet-like geological activity, yet have not cleared their orbital neighborhoods. Sedna is bright enough, and therefore large enough, that it is expected to be in hydrostatic equilibrium. Hence, astronomers generally consider Sedna a dwarf planet. The list of dwarf planets recognized by the IAU has not changed since 2006, even though Sedna is considered to be one by most astronomers. Besides its physical classification, Sedna is also categorized according to its orbit. The Minor Planet Center, which officially catalogs the objects in the Solar System, designates Sedna only as a trans-Neptunian object (as it orbits beyond Neptune), as does the JPL Small-Body Database. The question of a more precise orbital classification has been much debated, and many astronomers have suggested that the sednoids, together with similar objects such as , be placed in a new category of distant objects named extended scattered disc objects (E-SDO), detached objects, distant detached objects (DDO), or scattered-extended in the formal classification by the Deep Ecliptic Survey. == Exploration ==

Sedna will come to perihelion around July 2076. It was calculated in 2011 that a flyby mission to Sedna could take 24.48 years using a Jupiter gravity assist, based on launch dates of 6 May 2033 or 23 June 2046. Sedna would be either 77.27 or 76.43 AU from the Sun when the spacecraft arrives near the end of 2057 or 2070, respectively. Research at the University of Tennessee has also examined the potential for a lander. == Notes ==