Sedna was discovered in 2003 by Mike Brown and Chad Trujillo, the same team who have also discovered the dwarf planets Eris, Makemake, Quaoar, Salacia, Orcus, and 2007 OR10. Named after the Inuit goddess of the sea who in legend lived at the bottom of the Arctic Ocean, Sedna has been an enigma since its discovery, as there is no proven explanation as to how it got where it is, and its distance makes it difficult to study.
Sedna currently hovers around 20th magnitude and will never get significantly brighter, putting it outside the visual range of pretty much all amateur telescopes. However, it is bright enough to be imaged with a large telescope and lots of exposure time; this has in fact been done by multiple amateurs (1) (2)
1. It is one of the reddest objects in the Solar System
Sedna is one of the reddest objects in the Solar System besides Mars. Unlike Mars, however, the dwarf planet’s coloring comes not from iron oxide (rust) but rather from tholins, an organic slush created over billions of years of bombardment by cosmic rays. Sedna’s surface is probably an upper layer of tholins covering a methane and water ice crust.
2. It is extremely far from the Sun
Sedna spends most of its time extremely far from the Sun – it happened to be discovered near its closest approach when it is the easiest to observe, which means there are probably many unobservable Sedna-like objects out there. Currently, Sedna is around 85 astronomical units from the Sun or about 3 times the distance of Neptune! However, 8 objects including Eris and 2007 OR10, as well as the Voyager and Pioneer probes, are farther from the Sun.
Even at its perihelion (closest approach to the Sun), Sedna is still about twice as far as Pluto, at about 76 astronomical units. This means Sedna spends the majority of its time in “interstellar space” outside the bubble of particles and extremely thin plasma known as the heliosphere.
Sedna’s distant perihelion means it cannot have been put there via a gravitational “slingshot” by any of the gas giant planets. We now know of many Sedna-like objects with similar orbits. It is possible that they are all the result of a close approach by another star shortly after the formation of the Solar System, but the leading explanation is that a fifth gas giant planet in a highly irregular orbit has been providing an unseen gravitational influence that tugs these objects into their current orbits.
3. It is very cold & inactive
As a result of being so far from the sun, most of the time the surface temperature on Sedna hovers slightly above absolute zero, so cold that any thin nitrogen atmosphere would remain almost always frozen. There is probably not much geological activity, and Sedna’s remoteness means there are probably few impact craters. As a result, Sedna’s surface may be quite smooth and uniform – probably one of the most pristine surfaces in the Solar System, untouched since its formation. For 200 years or so around perihelion (which it is heading for in 2076), Sedna does warm up to above 35 degrees Kelvin and could host an atmosphere, however, none has been detected at this time.
Oddly enough, Sedna could still have a subsurface ocean, kept warm by the residual heat of its formation combined with the decay of radioactive elements(!) in its core. However, determining this would require at least a flyby probe if not a near-impossible orbiting mission.
4. It is a member of the Oort Cloud (sort of)
Sedna is a member of the Inner Oort cloud or Hills Cloud, a very sparse torus-shaped cluster of icy bodies beyond the outer reaches of the Kuiper Belt. While the main Oort Cloud is a massive spherical shell of comets which could stretch over a light-year out from the Sun, the Hills Cloud seems to consist of larger and rockier bodies like Sedna. As I’ve mentioned previously, Sedna and objects like it are probably pristine samples of the Solar System’s earliest days.
5. It is evidence for a ninth planet
Because of the highly elongated orbit that never takes it very close to the Sun, Sedna cannot possibly have been put in its current position by gravitational slingshots by the gas giant planets. Originally, it was thought that perhaps Sedna was either pulled outwards due to the gravitational tug of a nearby star or that maybe it was captured from another solar system entirely. But with at least ten objects with orbits like Sedna, the probability of something like this occurring repeatedly is low.
6. It is the largest dwarf planet without a moon
Most of the large dwarf planets such as Pluto, Eris, and others have at least one moon – Sedna does not. It is possible that one has simply evaded discovery, but for now, it is more likely than not that Sedna lacks a moon of any sort. Without a moon, scientists cannot determine the mass of Sedna and its exact size is difficult to determine accurately – estimates range from a diameter of 915 to 1160 kilometers in diameter, about half the size of Pluto.