You may have heard a lot about dwarf planets recently. This new category of celestial bodies was the result of a debate caused by the discovery of Eris. Sedna, discovered in 2003 and also known as 2003 VB12, is most likely a dwarf planet. One reason why astronomers are reluctant to definitively place it in that category is because it is so far away that it is difficult to observe. The object is 90 astronomical units (AU) from the Sun – an astronomical unit is the distance from the Earth to the Sun, which is approximately 93 million miles. Sedna does have a highly elliptical orbit, which means that it ranges from 76 AU to 975 AU.
Sedna is fascinating for a number of reasons. It is the furthest object from the Sun discovered as of yet. It is located in a hypothetical region called the Oort Cloud, which astronomers believe is filled with comets and other icy celestial objects. In fact, Sedna was the first object discovered in the Oort Cloud. Astronomers think that Sedna ist he fifth largest trans-Neptunian object (TNO) – and dwarf planet – after Eris, Pluto, Makemake, and Haumea. Sedna appears to be almost as red as Mars, which some astronomers believe is caused by hydrocarbon or tholin. Estimations on how long it takes Sedna to orbit the Sun vary, although it is more than 10,000 years. Some astronomers calculate the orbital period as more than 12,000 years long. Although astronomers believed at first that Sedna had a satellite, they have not been able to prove it. Sedna, and the entire Oort Cloud, is freezing at temperatures below 33 Kelvin (-240.2°C).
Sedna was named after a goddess, although it was not a Greek or Roman one. In Inuit mythology, Sedna was the goddess of the sea who was once a mortal but became immortal when she drowned in the sea. She is thought to live at the bottom of the Arctic Ocean and protect the creatures of the sea. The astronomers who discovered Sedna also recommended to the International Astronomical Union (IAU) that all the objects in the Oort Cloud should be named after figures in arctic mythology. Considering the frigid temperatures in the Oort Cloud, this proposition makes a lot of sense.
Astronomers believe that they will find more objects in the Oort Cloud in years to come, especially as telescopes and imaging equipment becomes more and more advanced. Most likely, we will also see Sedna officially christened a dwarf planet by the IAU.
Astronomy Cast has an episode on Pluto and the icy outer Solar System.