90377 Sedna is a trans-Neptunian object discovered in 2003, which as of 2011 was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units (32 times Neptune’s distance), making it one of the most distant known objects in the Solar System other than long-period comets.[b][f]
It is thought to be “nearly certainly” a dwarf planet, though the IAU has not formally designated it as such. Though roughly two-thirds the size of Pluto, its distance from the Sun makes determining its shape, and thus demonstrating that it is in hydrostatic equilibrium, difficult. 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 in the Solar System.
Sedna’s exceptionally long and elongated orbit, taking approximately 11,400 years to complete, and distant point of closest approach to the Sun, at 76 AU, have led to much speculation as to its origin. The Minor Planet Center currently places Sedna in the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune. However, this classification has been contested, as Sedna never comes close enough to Neptune to have been scattered by it, leading some astronomers to conclude that it is in fact the first known member of the inner Oort cloud. Others speculate that it might have been tugged into its current orbit by a passing star, perhaps one within the Sun’s birth cluster, or even that it was captured from another star system. Another hypothesis suggests that its orbit may be evidence for a large planet beyond the orbit of Neptune. Astronomer Michael E. Brown, co-discoverer of Sedna and the dwarf planets Eris, Haumea, and Makemake, believes it to be the most scientifically important trans-Neptunian object found to date, as understanding its unusual orbit is likely to yield valuable information about the origin and early evolution of the Solar System.