The Dwarf Planet Sedna

Facts About Sedna, the Distant Dwarf Planet

Sedna is a red world, like Mars. The Sun is very distant.
Sedna is a red world, like Mars. The Sun is very distant. Anne Helmenstine

Way past the orbit of Pluto, there's an object orbiting the Sun in a highly eccentric orbit. The object's name is Sedna and it's probably a dwarf planet. Here's what we know about Sedna so far.

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 if 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.

Sedna Facts and Figures

MPC Designation: Formerly 2003 VB12, officially 90377 Sedna

Discovery Date: November 13, 2003

Category: trans-Neptunian object, sednoid, possibly a dwarf planet

Aphelion: about 936 AU or 1.4×1011 km

Perihelion: 76.09 AU or 1.1423×1010 km

Eccentricity: 0.854

Orbital Period: about 11,400 years

Dimensions: estimates range from about 995 km (thermophysical model) to 1060 km (standard thermal model)

Albedo: 0.32

Apparent Magnitude: 21.1