Sedna (dwarf planet)

Lore & Background

Sedna was discovered on 14 November 2003 as part of a survey using the Samuel Oschin telescope at Palomar Observatory. Its slow movement initially masked its presence, leading Brown to nickname it 'The Flying Dutchman'. The name Sedna was chosen after the Inuit goddess of the sea, partly because Brown mistakenly thought the Inuit were the closest polar culture to his home in Pasadena. The name was announced before official numbering, causing some controversy among amateur astronomers, but was accepted by the IAU in September 2004.

Sedna's surface, revealed by spectroscopic analysis, is a mixture of solid ices of water, carbon dioxide, and ethane, along with reddish tholins derived from methane. It is not expected to have a substantial atmosphere. Its orbit is one of the widest and most elliptical known, with an aphelion of 937 AU and a perihelion of 76.2 AU. As of 2026, Sedna is 83.0 AU from the Sun. Its rotation period is about 10 hours, though an 18-hour period is considered unlikely.

Initially classified as a member of the scattered disc, Sedna's distant perihelion led some astronomers to consider it the first known member of the inner Oort cloud. It has since become the prototype of the sednoids, objects with highly eccentric orbits and very distant perihelia. Brown has argued that its unusual orbit could provide information on the early evolution of the Solar System, possibly perturbed by a close gravitational encounter with the hypothetical Planet Nine.

Reader's Guide

Sedna's significance lies in its extreme orbit, which challenges models of Solar System formation and dynamics. Its perihelion at 76.2 AU is too distant to have been caused by gravitational scattering from Neptune, suggesting an origin in the inner Oort cloud or perturbation by an unseen planet. As the prototype of the sednoids, Sedna has opened a new class of objects that may hold clues to the early Solar System. Its lack of known moons means its mass and density remain unknown, but its size suggests it could have undergone differentiation and possibly harbor a subsurface liquid ocean. Sedna's discovery also sparked debate about naming protocols and the classification of distant objects. Its orbit, taking 11,400 years, makes it one of the coldest places in the Solar System, with temperatures never exceeding −240 °C at aphelion. The object remains a key target for understanding the outer reaches of the Solar System and the potential existence of Planet Nine.

Did You Know?

• Sedna takes approximately 11,400 years to complete one orbit around the Sun.
• Its aphelion is 937 AU, about 19 times farther than that of Pluto.
• The name Sedna comes from the Inuit goddess of the sea, chosen partly because it was easily pronounceable by English speakers.