90377 Sedna is a large planetoid in the outer reaches of the Solar System that was, as of 2012, about three times as far from the Sun as Neptune. Spectroscopy has revealed that Sedna's surface composition is similar to that of some other trans-Neptunian objects, being largely a mixture of water, methane and nitrogen ices with tholins. Its surface is one of the reddest among Solar System objects. It is most likely a dwarf planet.
|Artist's conception of the surface of Sedna, with the Milky Way, Antares, the Sun and Spica above|
Sedna's orbit is extremely elongated, which is quite different from the circular planetary orbits that feature in our solar system. Brown told Space.com in 2011 that it was likely due to a close encounter.
"So the zeroth-order piece of information is that, somewhere out there, something perturbed Sedna, and that thing is no longer there. Now, that thing could've been another planet; it could have been a star that came close to the sun; it could have been a lot of stars, if the sun was born in a cluster," Brown said.
|Sedna as seen by a telescope|
In 2014, astronomers announced the discovery of 2012 VP113 (nicknamed "Biden") that is also believed to be in the inner Oort cloud. The new object's orbit is also extremely elongated, suggesting that perhaps the same thing perturbed both Sedna and 2012 VP113.
How far away is Sedna?Sedna is the most distant solar system object ever discovered. It is twice as far from the sun as any other solar system object and three times farther than Pluto or Neptune. Standing on the surface of Sedna, you could block the entire sun with the head of a pin held at arm's length.
Even more interestingly, the orbit of Sedna is extreme elliptical, in contrast to all of the much closer planets, and it takes 10,500 years to circle the sun.
Is Sedna a Kuiper belt object?NO. Sedna never enters the region of the Kuiper belt. The Kuiper belt is an icy asteroid belt just beyond Neptune. Extremely strong evidence shows that it has a rather sharp edge at 50 AU. Sedna never comes close than 76 AU. Calling Sedna an inner Oort cloud object makes much more sense.
There are some KBOs that go very far from the sun like Sedna does, but they all have closest approach at about 35-45 AU. Sedna is special because it doesn't come any closer than 75 AU to the sun. We believe that this is because of the effects of passing stars, as described above.
A second speculative explanation for Sedna's orbit is that a larger body, perhaps Mars-sized or larger could exist at around 70 AU in a circular orbit and could have caused Sedna to get thrown into its strange orbit. If such a planet existed, we would likely have already found it in our survey, though there are still a few places left to hide.