Sedna

90377 Sedna, or simply Sedna, is a large planetoid in the outer reaches of the Solar System that was, as of 2020, at a distance of about 85 astronomical units (1.27×1010 km; 7.9×109 mi) from the Sun, about 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. It is a possible dwarf planet. Sedna is approximately tied with 2002 MS4 and 2002 AW197 as the largest planetoid not known to have a moon.

For most of its orbit, it is even farther from the Sun than at present, with its aphelion estimated at 937 AU (31 times Neptune's distance, or about 1.5% of a light-year), making it one of the most distant-known objects in the Solar System other than long-period comets.

Sedna has an exceptionally long and elongated orbit, taking approximately 11,400 years to complete and a distant point of closest approach to the Sun at 76 AU. These facts have led to much speculation about its origin. The Minor Planet Center currently places Sedna in 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 a known planet, leading some astronomers to informally refer to it as the first known member of the inner Oort cloud. Others speculate that it might have been tugged into its current orbit by a passing star, perhaps one within the Sun's birth cluster (an open cluster), or even that it was captured from another star system. Another hypothesis suggests that its orbit may be evidence for a large planet beyond the orbit of Neptune. 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.