page_title: 90377 Sedna
section_title: Classification
hierarchical_section_title: 90377 Sedna / Classification
caption_attribution_description: English: A screenshot of the orbits of some of the most distant objects in the Solar System.
context_page_description: 90377 Sedna, or simply Sedna, is a large planetoid in the outer reaches of the Solar System that was, as of 2020, at a distance of about 85 astronomical units from the Sun, about three times as far as Neptune. Spectroscopy has revealed that Sedna's surface composition is similar to those of some other trans-Neptunian objects, being largely a mixture of water, methane, and nitrogen ices with tholins. Its surface is one of the reddest among Solar System objects. It is a possible dwarf planet. Sedna is approximately tied with 2002 MS4 and 2002 AW197 as the largest planetoid not known to have a moon.
  For most of its orbit, it is even farther from the Sun than at present, with its aphelion estimated at 937 AU, making it one of the most distant-known objects in the Solar System other than long-period comets.
  Sedna has an exceptionally long and elongated orbit, taking approximately 11,400 years to complete and a distant point of closest approach to the Sun at 76 AU. These facts have led to much speculation about its origin. The Minor Planet Center currently places Sedna in the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune.
context_section_description: The Minor Planet Center, which officially catalogs the objects in the Solar System, classifies Sedna as a scattered object. This grouping is heavily questioned, and many astronomers have suggested that it, together with a few other objects (e.g. 2000 CR105), 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.
  The discovery of Sedna resurrected the question of which astronomical objects should be considered planets and which should not. On 15 March 2004, articles on Sedna in the popular press reported that a tenth planet had been discovered. This question was answered under the International Astronomical Union definition of a planet, adopted on 24 August 2006, which mandated that a planet must have cleared the neighborhood around its orbit. Sedna has a Stern–Levison parameter estimated to be much less than 1, and therefore cannot be considered to have cleared the neighborhood, even though no other objects have yet been discovered in its vicinity. To be a dwarf planet, Sedna must be in hydrostatic equilibrium. It is bright enough, and therefore large enough, that this is expected to be the case, and several astronomers have called it one.
caption: Sedna compared to some other very distant orbiting bodies [f]
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