Sedna (minor-planet designation 90377 Sedna) is a possible dwarf planet and ETNO in the outermost reaches of the Solar System discovered in 2003, orbiting the Sun very well beyond any other planet to be affected by them in any way. Sedna is insanely red like Mars.
Spectroscopy has revealed that Sedna's surface composition is largely a mixture of water, methane, and nitrogen ices with tholins, similar to those of some other trans-Neptunian objects.
Sedna, within estimated uncertainties, is tied with Ceres as the largest dwarf planet / planetoid not known to have a moon. It has a diameter of approximately 1,000 km, nearly the size of Saturn's Tethys. Due to not having moons we do not know its mass.
Sedna's orbit is one of the largest in the Solar System other than those of long-period comets, with its aphelion (farthest distance from the Sun) estimated at 937 astronomical units (AU). This is 31 times Neptune's distance from the Sun, and well beyond the closest portion of the heliopause, which defines the outer boundary of interplanetary space. As of 2022, Sedna is near perihelion, its closest approach to the Sun, at a distance of 84 AU (12.6 Tm), almost three times farther than Neptune.
Sedna's orbit is one of the widest known in the Solar System. Its aphelion, the farthest point from the Sun in its orbit, is located 937 AU away from the Sun. This is 19 times that of Pluto, Sedna spends most of its existence outside of the heliopause (120 AU), the boundary beyond which the influences of particles from interstellar space dominate those from the Sun. Sedna's orbit is also one of the most elliptical and narrow known, with an eccentricity of ~0.85. This implies that its perihelion at 76 AU is around 12.3 times closer than aphelion.
As of February 2025, Sedna is 83.20 AU (12.45 billion km) from the Sun, approaching perihelion at ~4.4 km/s, and 2.5 times as far away as Neptune. The dwarf planets Eris and Gonggong are currently farther away from the Sun. A transfer window for a probe fly-by in 2029 utilizing a gravitational assist from Jupiter was proposed, taking 25 years to travel to the dwarf planet 80 AU away.
Sedna has an exceptionally elongated orbit, and takes approximately 11,400 years to return to its closest approach to the Sun at a distant 76 AU.
The IAU initially considered Sedna a member of the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune. However, several astronomers contested this classification, because its perihelion is too large for it to have been scattered by any of the known planets. This has led some astronomers to informally refer to it as the first known member of the inner Oort cloud. It is the prototype of a new orbital class of object, the Sednoid, which also include Biden, Goblin and 2023 KQ14.
Astronomer Michael E. Brown, co-discoverer of Sedna, believes that understanding Sedna's unusual orbit could yield information about the origin and early evolution of the Solar System. It might have been perturbed into its orbit by a one or more stars within the Sun's birth cluster, or possibly it was captured from the planetary system of another star. The clustering of the orbits of Sedna and similar objects is speculated to be evidence for a planet beyond the orbit of Neptune.
History
Sedna, provisionally designated 2003 VB12 was discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) on 14 November 2003. On that day, an object was observed to move by 4.6 arcseconds over 3.1 hours relative to stars, which indicated that it was about 100 AU away. Follow-up observations were made in November–December 2003 with the SMARTS at Cerro Tololo Inter-American Observatory in Chile, the Tenagra IV telescope in Arizona, and the Keck Observatory on Mauna Kea. 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.
Sedna was initially nicknamed "The Flying Dutchman", or "Dutch" by Michael Brown 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 IAU's 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 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.