Sedna

Trans-Neptunian object · Dwarf-planet candidate (detached object) · orbits Sun

Sedna is one of the loneliest worlds known: a reddish body roughly 1000 km across on an 11,400-year orbit that never brings it closer to the Sun than about 76 AU - far beyond Neptune’s reach - and carries it out past 900 AU. It is named for the Inuit goddess of the sea, who lives at the bottom of the cold Arctic Ocean.

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Quick facts

It reaches perihelion in 2076 - the one close approach in 11,400 years, and our one chance to send a probe.
It is among the reddest objects in the Solar System, likely coated in radiation-cooked organic “tholins”.
Near aphelion, over 900 AU out, sunlight is nearly a million times fainter than on Earth.

An orbit nothing present can explain

Every planet, asteroid and comet we know was put on its path by gravity we can point to. Not Sedna: it never comes near Neptune, so Neptune cannot have scattered it, yet it is far too tightly bound for passing stars to stir it today. Its orbit looks like a fossil - imprinted billions of years ago by a star sweeping through the Sun’s birth cluster, or by a distant unseen mass still waiting to be found.

Physical characteristics

Diameter	1,000 km
Rotation period	10.3 h
Geometric albedo	0.32
Known moons	0

Atmosphere and composition

Atmosphere	None detected.
Composition	Uncertain - its intensely red surface suggests organic tholins over ices of water, methane and nitrogen.

Discovery

Discovered by	Mike Brown, Chad Trujillo & David Rabinowitz
Year	2003
How	Palomar Observatory 48-inch Samuel Oschin Telescope

What is still uncertain

Sedna’s diameter (~1000 km) is a thermal-model estimate - it has never been resolved by any telescope, and its mass is unknown.