Sedna is a dwarf planet in the outer reaches of the Solar System that is currently in the innermost part of its orbit; as of 2021 it is 84 astronomical units from the Sun, almost three times farther than 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. To within estimated uncertainties, Sedna is tied with Ceres as the largest planetoid not known to have a moon.

For nearly all of its orbit, Sedna is even farther from the Sun than at present: its orbit is one of the largest in the Solar System other than those of long-period comets, with its aphelion estimated at 937 AU. 937 AU is 31 times Neptune's distance, 1.5% of a light-year (or 5.5 light-days) and well beyond the closest portion of the heliopause, which defines the boundary of interstellar space. (Eris and Gonggong are currently further from the Sun, at 96.0 AU and 88.3 AU, respectively, than Sedna at 84.7 AU (As of 2019), because Sedna is near perihelion.)

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 Minor Planet Center currently categorized Sedna as part of the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune. This classification has been contested because its perihelion is too large for it to have been scattered by any of the known planets, leading some astronomers to informally refer to it as the first known member of the inner Oort cloud. It is also the prototype of a new orbital class of object, the sednoids.

Astronomer Michael E. Brown, co-discoverer of Sedna and numerous other possible dwarf planets, thinks that it is the most scientifically important trans-Neptunian object found to date, because understanding its unusual orbit is likely to yield valuable information about the origin and early evolution of the Solar System. It may have been tugged into its current orbit by a passing star, or perhaps by several stars within the Sun's birth cluster, or it may even have been captured from the planetary system of another star. There is also speculation that the clustering of the orbits of Sedna and similar objects may be evidence for a planet beyond the orbit of Neptune.

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