Description

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, a moon of 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.