The Discovery of Sedna
Sedna was co-discovered on November 14, 2003 by Michael E. Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale). Brown was also a co-discoverer of the dwarf planets Eris, Haumea, and Makemake. The team announced the name "Sedna" before the object had been numbered, which was not proper protocol for the International Astronomical Union (IAU), but didn't raise objections. The world's name honors Sedna, the Inuit sea goddess who lives at the bottom of the icy Arctic Ocean. Like the goddess, the celestial body is very far away and very cold.

Is Sedna a Dwarf Planet?
It's likely Sedna is a dwarf planet, but uncertain, because it's so far away and hard to measure. In order to qualify as a dwarf planet, a body must have enough gravity (mass) to assume a rounded shape and may not be a satellite of another body. While the plotted orbit of Sedna indicates its not a moon, the world's shape is unclear.

What We Know About Sedna
Sedna is very, very distant! Because it's between 11 and 13 billion kilometers away, its surface features are a mystery. Scientists do know it's red, much like Mars. A few other distant objects share this distinctive color, which could mean they share a similar origin. The extreme distance of the world means if you viewed the Sun from Sedna, you could blot it out with a pin. However, that pinprick of light would be bright, about 100 times brighter than the full moon viewed from Earth. To put this into perspective, the Sun from Earth is around 400,000 times brighter than the Moon.

The size of the world is estimated to be about 1000 kilometers, which makes it about half the diameter of Pluto (2250 km) or around same size as Pluto's moon, Charon. Originally, Sedna was believed to be much larger. It's likely the size of the object will be revised again as more is known.

Sedna is located in the Oort Cloud, a region containing many icy objects and the theoretical source of many comets.

It takes a long time for Sedna to orbit the Sun, longer than any other known object in the solar system. Its 11000 year cycle is so long partially because it's so far out, but also because the orbit is highly elliptical rather than round. Usually, oblong orbits are due to a close encounter with another body. If an object either impacted Sedna or drew close enough to affect its orbit, it's no longer there. Likely candidates for such an encounter include a single passing star, an unseen planet out beyond the Kuiper belt, or a young star that was with the Sun in a stellar cluster when it formed.

Another reason a year on Sedna is so long is because the body moves relatively slowly around the Sun, about 4% as fast as the Earth moves.

While the present orbit is eccentric, astronomers believe Sedna likely formed with a near-circular orbit that was disrupted at some point. The round orbit would have been necessary for particles to clump together or accrete to form a rounded world.

Sedna has no known moons. This makes it the largest trans-Neptunian object orbiting the Sun that doesn't have its own satellite.

Speculations About Sedna
Based on its color, Trujillo and his team suspect Sedna may be coated with tholin or hydrocarbons formed from solar irradiation of simpler compounds, like ethane or methane. The uniform color could indicate Sedna doesn't get bombarded with meteors very often. Spectral analysis indicates the presence of methane, water, and nitrogen ices. The presence of water could mean Sedna had a thin atmosphere. Trujillo's model of the surface composition suggests Sedna is coated with 33% methane, 26% methanol, 24% tholins, 10% nitrogen, and 7% amorphous carbon.

How cold is Sedna? Estimates place a hot day at 35.6 K (−237.6 °C). While methane snow may fall on Pluto and Triton, it's too cold for organic snow on Sedna. However, if radioactive decay heats the interior of the object, Sedna could have a subsurface ocean of liquid water.

Sources
Malhotra, Renu; Volk, Kathryn; Wang, Xianyu (2016). "Corralling a distant planet with extreme resonant Kuiper belt objects". The Astrophysical Journal Letters. 824 (2): L22. doi:10.3847/2041-8205/824/2/L22
Mike Brown; David Rabinowitz; Chad Trujillo (2004). "Discovery of a Candidate Inner Oort Cloud Planetoid". Astrophysical Journal. 617 (1): 645–649. doi:10.1086/422095

Sedna, or simply Sedna, is a dwarf planet or large planetoid in the outer reaches of the Solar System that was, as of 2021, at a distance of about 84 astronomical units (1.26×10¹⁰ km; 0.00041 pc) from the Sun, about three times farther than Neptune.

Orbit: Discovered in 2004 and named after the Inuit goddess of the sea, Sedna (also known as 90377 Sedna) is 800 miles wide—twice the size of Biden. It gets to within 76 au of the Sun, and to 937 au away from it during its 11,400-year orbit.

A team led by astronomer Mike Brown discovered 90377 Sedna in late 2003. Provisionally named 2003 VB12, the object later received the name Sedna from the International Astronomical Union, after the Inuit goddess of the sea. From the start, Sedna was an odd-ball.

The distance of Asteroid 90377 Sedna (2003 VB12) from Earth is currently 12,640,704,506 kilometers, equivalent to 84.497891 Astronomical Units .