90377 Sedna

Sedna (minor-planet designation 90377 Sedna) is a dwarf planet in the outermost reaches of the inner Solar System, orbiting the Sun beyond the orbit of Neptune. Discovered in 2003, the planetoid's surface is one of the reddest known among Solar System bodies. Spectroscopy has revealed Sedna's surface to be mostly a mixture of the solid ices of water, methane, and nitrogen, along with widespread deposits of reddish-colored tholins, a chemical makeup similar to those of some other trans-Neptunian objects. Within the range of uncertainties, it is tied with the dwarf planet Ceres in the asteroid belt as the largest dwarf planet not known to have a moon. Its diameter is roughly 1,000 km (most likely in between those of Ceres and Saturn's moon Tethys). Owing to its lack of known moons, the Keplerian laws of planetary motion cannot be employed for determining its mass, and the precise figure as yet remains unknown.

Sedna's orbit is one of the widest known in the Solar System. Its aphelion, or its farthest point from the Sun in its elliptical orbit, is located 937 astronomical units (AU) away. This is some 31 times the distance of Neptune's aphelion, and 19 times that of Pluto, sending most of its highly elongated orbit well beyond the heliopause, the boundary beyond which the influence of particles from interstellar space dominates over that of the Sun. Sedna's orbit is also one of the most narrow and elliptical discovered, with an eccentricity of 0.8496. This means that its perihelion, or point of closest approach to the Sun, at 76 AU is around 12.3 times closer than its aphelion. At perihelion, Sedna is only 55% further than Pluto's aphelion. As of January 2024, Sedna is near perihelion, 83.55 AU (12.50 billion km) from the Sun, and 2.8 times farther away than Neptune. The dwarf planets Eris and Gonggong are presently farther away from the Sun than Sedna. It is suggested that an exploratory fly-by mission to Sedna near its perihelion through a Jupiter gravity assist could be completed in 24.5 years.

Due to its exceptionally elongated orbit, the dwarf planet takes approximately 11,400 years, over 11 millennia, to return to the same point in its orbit around the Sun. The IAU initially considered Sedna to be a member of the scattered disc, a group of objects sent into high-eccentricity orbits by the gravitational influence of Neptune. Several astronomers who worked in the associated field contested this classification, however, as even its perihelion is far too distant for it to have been scattered by any of the currently known planets. This has led some astronomers to informally refer to it as the first known member of the inner Oort cloud. The dwarf planet is also the prototype of a new orbital class of objects named after itself, the sednoids, which include 2012 VP113, Leleākūhonua, and 2021 RR₂₀₅, all celestial bodies with extremely elongated orbits.

The astronomer Michael E. Brown, co-discoverer of Sedna, believes that studying Sedna's unusual orbit could yield valuable information on the origin and early evolution of the Solar System. It might have been perturbed into its orbit by one or more stars within the Sun's birth cluster, or captured from a nearby wandering star, or to have been sent into its present orbit through a close gravitational encounter with the hypothetical 9th planet, some time during the solar system's formation. The statistically unusual clustering to one side of the solar system of the aphelions of Sedna and other similar objects is speculated to be the evidence for the existence of a planet beyond the orbit of Neptune, which would by itself orbit on the opposing side of the Sun.