90377 Sedna

Sedna (minor-planet designation 90377 Sedna) is a dwarf planet in the outermost reaches of the Solar System discovered in 2003. Spectroscopy has revealed that Sedna's surface composition is largely a mixture of water, methane, and nitrogen ices with tholins, similar to those of some other trans-Neptunian objects. Its surface is one of the reddest among Solar System objects. Sedna, within estimated uncertainties, is tied with as the largest planetoid not known to have a moon. It has a diameter of approximately 1,000 km (most likely between the sizes of the asteroid Ceres and Saturn's moon Tethys), with an unknown mass. Sedna's orbit is one of the largest in the Solar System other than those of long-period comets, with its aphelion (farthest distance from the Sun) estimated at 937 astronomical units (AU). This is 31 times Neptune's distance from the Sun, and well beyond the closest portion of the heliopause, which defines the outer boundary of interplanetary space. , Sedna is near perihelion, its closest approach to the Sun, at a distance of , almost three times farther than Neptune. The dwarf planets and are presently further from the Sun than Sedna. An exploratory fly-by mission to Sedna at perihelion could be completed in 24.5 years using a Jupiter gravity assist. 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 IAU initially considered Sedna a member of the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune. However, several astronomers contested this classification, because its perihelion is too large for it to have been scattered by any of the known planets. This has led some astronomers to informally refer to it as the first known member of the inner Oort cloud. It is the prototype of a new orbital class of object, the sednoids, which include and Leleākūhonua. Astronomer Michael E.