written by: Arlene McKanic•edited by: RC Davison•updated: 6/28/2011
Sedna, a very distant planetoid with a very long, eccentric orbit around the Sun, has much to tell us about the origins of the Solar System.
slide 1 of 4
The Far Away One
Of all the planetoids, dwarf planets, asteroids, comets and other Solar System curiosities that we’ve found this century, Sedna has the distinction of being the farthest object yet discovered that still orbits the Sun. Sedna’s about 8 billion miles from Earth. This planetoid's discovery was announced on November 14, 2003. She’s classified as a planetoid because she’s too small to be a planet, though she’s still the second largest object in the Solar System to be discovered since Pluto was discovered in 1930.
slide 2 of 4
Mythological Origins and Some Facts
As astronomers have broken away from naming new planetoids after Greek and Roman gods and goddesses, Sedna, as cold as she is distant, is named for the rageful Inuit sea goddess who’s supposed to live at the bottom of the Arctic ocean. Her hands and fingers gave birth to all sea creatures. The planetoid was named by the astronomers who discovered her: Mike Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale.
Sedna, originally named 2003VB12, is so far away from the Sun that her very large elliptical orbit takes 10,500 to 11,400 years to complete. It’s believed that her surface temperature is about -400°F. Measurements by infrared telescope, which is the most accurate way to measure an object so distant, suggests that Sedna has a diameter between around 777 and 1118 miles. This makes Sedna smaller than Pluto, which is 1400 miles in diameter.
At first scientists believed Sedna had a very slow rotation, about every 40 days. Only Mercury and Venus rotate more slowly, Mercury at 176 days and Venus at 243 days. Some scientist believed that such slow rotation meant that Sedna had a moon whose gravity slowed her down, but no moon was found. After closer observation it was determined that Sedna’s rotation was only about 10 hours, which is closer to the rotational periods of other planetoids and asteroids of her size.
Sedna is also about as red as Mars, but no one knows why. No one knows just what Sedna is made of, though it’s thought that the surface has water ice, methane ice, nitrogen ice, organics and particles called tholins, strange molecules created from organic compounds being exposed to ultra violet radiation. Tholins are found on very cold bodies and usually have a reddish color. Sedna might have liquid water beneath her surface as well.
slide 3 of 4
From the Oort Cloud?
Sedna is thought to dwell in the inner, sunward regions of the Oort cloud, a cloud of chunks of ice and freezing cold worldlets that’s believed to be the origin of comets. If so, Sedna is the first object ever observed in the Oort cloud, whose existence till now has only been hypothetical. Comets arise when, from time to time, something knocks one of these ice bodies out of their orbit and sends it hurtling in toward the Sun. The same sort of phenomenon -- maybe a runaway star or planet? -- probably caused Sedna’s strange, elliptical orbit as well. Scientist also speculate that companion stars to the Sun, present during its birth, could have caused Sedna to take up the orbit she now has. Such a theory means that the Sun was part of a cluster of newborn stars that drifted away from each other over eons and probably formed their own stellar systems. This has great implications for scientists' understanding of how our Solar System was formed and why Sedna is an important object in the solar system. What if the Sun hadn't been born alone but had siblings? Where are they? What happened to them? Are they still influencing our Solar System?
It’s also possible that Sedna isn’t of our Solar System at all, but was captured by the Sun’s gravity from another stellar system passing through the Sun’s neighborhood. Other scientists blame Sedna’s orbit on an as yet undiscovered stellar companion of the Sun called Nemesis, but there’s no indication that Nemesis exists. Others believe that her orbit is the result of the disturbance caused by the gravity of an unknown planet between 1,000 and 5,000 AU’s out from the Sun, but as with Nemesis, this planet hasn’t been found.
At her closest Sedna is about 75 AU or Astronomical Units from the Sun. An Astronomical Unit is the distance Earth is from the Sun, which is about 93 million miles. Because of this, scientists now wonder if the Oort cloud, at first believed to be extremely far out in the Solar System, perhaps even as much as a light year, is closer than was once thought. Sedna will be at her closest to Earth in about 72 years. But she’ll still be extremely far from us.