Sedna was not the only object close to Pluto's size to be discovered in the early 2000s. Brown's team also found Quaoar, another dwarf planet candidate, and Eris.

Sedna appears unusual in other ways besides its orbit. First and foremost, it is one of the largest known “minor planets,” with an estimated size of 1,000 miles compared to Pluto’s 1,400 miles.

Sedna, the Solar System's farthest known object, does not have a moon, puzzled astronomers have revealed. Its slow spin was thought to be due to the gravity of a small, companion body. Researchers ...