A tiny and distant planetoid that pursues an extreme orbit out into the depths of the Solar System, far from the Sun, and potentially belonging to the group of bodies known as dwarf planets. Discovered in 2003, Sedna is still something of mystery. It is currently classified as part of the group of bodies designated as the 'scattered disc', objects thrown out into elongated orbits by the planet Neptune, though in Sedna's case this classification is controversial.
Sedna's orbit is exceptionally eccentric in nature: at its most distant, Sedna can reach a distance of some 936 AU from the Sun, or about 1% of a light year. At present, however, it is approaching its perihelion point, its closest approach to the heart of the Solar System. This will bring it to within some 76 AU of the Sun, a point still immensely distant (far beyond the orbit of Pluto) which it will probably reach in the year 2076. In the skies of Earth, Sedna is presently passing through Taurus, in the direction of Orion and Gemini.
The origins of Sedna's extreme orbit are still mysterious, and a range of possibilities remain open. Sedna appears to have formed on the outer fringes of the Solar System following a far less eccentric orbit, an orbit that was then perturbed by another body. The identity of that body is unclear - if indeed it existed at all. It may have been another star during the early development of the Solar System, or perhaps even a more massive planet yet to be discovered in a distant orbit of the Sun.
Sedna is a tiny body, even by comparison with similar objects, with an estimated diameter of a little under 1,000 km. By comparison, Pluto has a diameter more than twice this value, and Earth's Moon is more three times Sedna's diameter. Early observations of Sedna soon after its discovery hinted that it might have an even tinier body orbiting it as a moon, but further investigation suggests that this is not in fact the case.
In common with similar planetoids, Sedna's surface is covered with frozen methane, water and nitrogen, with the methane in particular creating a distinct reddish hue. These frigid conditions explain Sedna's name, which comes from an Inuit sea-goddess believed to dwell in the frozen depths of the northern oceans.