90377 Sedna

90377 Sedna, or simply Sedna, is a dwarf planet in the outer reaches of the Solar System that was, as of 2014, at a distance of about 85 AU (12,700,000,000 km) from the Sun, about three times farther than Neptune. The size is 1490 km and the 2nd smallest dwarf planet. 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 red corals. Its surface is one of the reddest among Solar System objects.

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, about 191 billion km, approximately 5.5 light-days), making it one of the most distant-known objects in the Solar System other than long-period comets. By comparison, the Sun's heliopause, which is one definition for the boundary of the Solar System, lies at around 121 AU from the Sun, according to measurements by the Voyager 2 spacecraft. This means that in certain points of its orbit, especially at aphelion, Sedna technically lies in interstellar space and can be considered an "interstellar object," although that term is usually used to refer to an object in interstellar space that is not gravitationally bound to a nearby star. Sedna has an exceptionally long and elongated orbit, taking approximately 11,400 years to complete with a point of closest approach to the Sun at a distant 76 AU.