Sedna

90377 Sedna is a trans-Neptunian object discovered in 2003, which currently lies about three times as far from the Sun as Neptune.

A picture of Sedna

Characteristics

Roughly two-thirds the size of Pluto, Sedna is theoretically large enough to be rounded by its own gravity, and thus would qualify as a dwarf planet under current definitions. However, its distance from the Sun makes determining its shape 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 solid nitrogen with tholins.