About the image. Drawn as a deep red icy ball. Sedna has never been seen as more than a point of light; its colour comes from measurements, but the surface markings are imagined.
On 14 November 2003 three astronomers, Mike Brown, Chad Trujillo and David Rabinowitz, found a slow-moving point of light far beyond Pluto. It was given the name Sedna, after the Inuit goddess of the sea, who lives in the cold depths of the Arctic Ocean. The name suited it: Sedna is one of the most distant and coldest known worlds in the Solar System.
An orbit no planet explains
Most objects beyond Neptune were placed on their orbits by Neptune’s gravity. Sedna was not. Even at its closest it stays 76 times as far from the Sun as Earth, more than twice Neptune’s distance, too far out for Neptune or any other known planet to have flung it there. At its farthest it swings out to about 937 AU. One trip around the Sun takes about 11 400 years.
Something else must have shaped that orbit. The possibilities include a star that passed close to the young Sun, the crowded cluster of stars in which the Sun was born, or an undiscovered planet far out in the Solar System. Sedna is one of the objects whose orbits inspired the search for Planet Nine.
Sedna is now heading towards its closest point to the Sun, which it will reach in 2076. That is why it is bright enough to study: for most of its long orbit it would be far too faint to find.
Red ice
Sedna is about 1 000 kilometres across, a little less than half the size of Pluto. It is one of the reddest objects known in the Solar System, almost as red as Mars. The colour comes from tholins, complex organic compounds made when sunlight and cosmic rays break down simple ices over billions of years.
In 2022 the James Webb Space Telescope took Sedna’s spectrum and found ethane, acetylene and ethylene: hydrocarbons formed from methane by radiation. Sedna has no known moon, so unlike many of its neighbours its mass has never been measured.
How far out it would be
Sedna goes around once every 11 390 years, at 506 AU from its star. Dashed circles show Jupiter, Saturn and Neptune around the Sun, on the same scale.
Earth days passing: 0
Gallery
A cratered, deep red world lit from the left by the Sun, which is only a small bright star at that distance, an impression of Sedna. AI-generated painting.A rough, rust-red frozen plain under a black sky crossed by the arc of the Milky Way, with a tiny figure standing on the horizon, an impression of the surface of Sedna. AI-generated painting.Sedna, 3D artist's impression, rendered from the interactive model.Sedna, 3D artist's impression, from another angle, rendered from the interactive model.Sedna, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2003Discovered on 14 November by Mike Brown, Chad Trujillo and David Rabinowitz.
2004Announced and named after the Inuit sea goddess.
2022JWST finds ethane, acetylene and ethylene on its surface.
2076Next closest approach to the Sun, at 76 AU.
Questions people ask
Is Sedna a planet?
No. It is generally considered a dwarf planet, about 1 000 km across.
How long is a year on Sedna?
About 11 400 Earth years, on an orbit that ranges from 76 to about 937 AU.
Why is Sedna red?
Its surface is covered in tholins, organic compounds made when radiation breaks down simple ices over billions of years.
Why is Sedna's orbit strange?
Its closest point to the Sun is too far out for Neptune or any known planet to have put it there. A passing star, the Sun's birth cluster or an unseen planet may be responsible.