The dwarf planet with the longest orbital period is Eris. It takes about 557 Earth years to complete one orbit around the Sun, making its “year” more than twice as long as Pluto’s.
Eris travels on a highly inclined, elongated orbit beyond Neptune. It belongs to the scattered disc, a distant region of icy objects associated with the Kuiper Belt. Its farthest distance from the Sun is roughly 97 astronomical units, while Earth’s average distance is 1 AU.
Eris was discovered in 2005 and initially appeared large enough to be called the Solar System’s possible tenth planet. Its discovery helped prompt the International Astronomical Union’s 2006 definition of “planet,” which placed Eris and Pluto in the dwarf-planet category.
A common mix-up is Sedna, whose orbit takes roughly 11,400 years. Sedna is often described as a dwarf-planet candidate, but it is not one of the five officially recognized dwarf planets. Among the recognized dwarf planets, Eris has the longest orbital period.