Sedna (goddess Inuit of the ice-cold oceans of the north pole), is the biggest trans-Neptunian object after Pluto and Eris. Sedna was discovered by Michael, E. Brown, Chadwick, Has. Trujillo and David L. Rabinowitz on November 14th, 2003, from the telescope Samuel Oschin du Mont Palomar in California. This object is three times farther of the Sun than a Pluto, the current limit of the solar system. The planetoid would measure between 1100 and 1800 km in diameter.

Between 30 and 50 UA of the Sun, are the bodies of what we call the Belt of Kuiper then, very beyond, at least 10 000 UA, those of the Cloud of Oort. Between both, it was not supposed to have there much... It is to 70 UA that was tracked down Sedna, in the great surprise of the astronomers. Thanks to numeric simulations, Scott Kenyon, and Benjamin Bromley, suggest that Sedna comes from another solar system of which he would have been extracted, there is more than 4 billion years.

Situated in an almost empty space, Sedna is more red and more brilliant than any object of the solar system. The scientists have not determined the reason of these unique characteristics yet. He could possess the small moon. In April, 2005 a more precise measure of the celestial body allowed to determine a complete rotation speed about 10 hours. At the time of its discovery, Sedna was the biggest object discovered in the solar system since the discovery of Pluto. Bigger objects ( dwarfish planets) were since discovered, as Eris. Sedna contributed to the new definition which returned the planets of the solar system for eight. Its orbit is very elliptic. its aphelia is approximately 942 UA of the Sun, its perihelia, that it should achieve en2076, is estimated at 76 UA. It makes a revolution in 11 486 years. According to David C. Jewitt, Sedna was not able to form there where she is: The disk proto planetary was too much fine in this place to engender an object of this size.

According to the scientist, Sedna formed either in the belt of Kuiper, or in the region of planets. It is later that it would have been ejected by a gravitational interaction, outside the orbit of Neptune. " A planet is a celestial body which is in orbit around the Sun, which possesses a sufficient mass so that its gravity takes it on the strengths of cohesion of the solid body and maintains it in hydrostatic balance ( spherical shape), and which eliminated any body moving on a close orbit ".
This definition was approved on August 24th, 2006, during the 26th General assembly of the UAI by a vote by a show of hands about 400 scientists and astronomers after ten days of discussions. In addition, the UAI created a new class of objects: the dwarfish planets the first members of which are a Pluto, Ceres and Eris.