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. Discovered in 2003, the frigid planetoid is one of the reddest known among Solar System bodies. Detailed spectroscopic analysis has revealed Sedna's surface to be a mixture of the solid ices of water (H₂O), carbon dioxide (CO₂), and ethane (C₂H₆), along with occasional sedimentary deposits of methane (CH₄)-derived, vividly reddish-colored organic tholins, a surface chemical makeup somewhat similar to those of other trans-Neptunian objects. Sedna has no detectable atmosphere, as its temperature is far too low for solids to volatilize. Within range of uncertainty, it is tied with the dwarf planet Ceres in the asteroid belt as the largest dwarf planet not known to have a moon. With a diameter of roughly 1,000 km, it is nearly the size of Tethys around Saturn. Owing to its lack of known moons, the Keplerian laws of planetary motion cannot be utilized for determining its mass, and the actual figure remains as yet unknown.

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 astronomical units (AU) away. This is some 19 times that of Pluto, leading to it spending most of its time well beyond 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 discovered, with an eccentricity of 0.8496. This implies that its perihelion, or point of closest approach to the Sun, at 76 AU is around 12.3 times as close as its 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 presently 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 (12 billion kilometers) distant.

Due to its exceptionally elongated orbit, the dwarf planet takes approximately 11,400 years to return to the same point in its orbit around the Sun. The International Astronomical Union (IAU) initially classified Sedna as a member of the scattered disc, a group of objects sent into high-eccentricity orbits by the gravitational influence of Neptune. However, several astronomers who worked in the associated field contested this classification 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 orbit class of objects named after itself, the sednoids, which include 2012 VP113 and Leleākūhonua, both celestial bodies with large perihelion distances and high eccentricities.

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 by a star within the Sun's birth cluster, or captured from a nearby wandering star, or have been sent into its present orbit through a close gravitational encounter with the hypothetical 9th planet, sometime 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.