Sedna (dwarf planet)

Sedna (minor-planet designation: 90377 Sedna) 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 Ceres 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. As of 2026, Sedna is 83.0 AU (12.4 billion km) from the Sun, 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 76.2 AU (11.4 billion km) 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.