That depends on who you ask. Pluto technically has the longest year; each Plutonian year is equal to 247.7 Earth years. However, some scientists no longer consider Pluto to be a planet, and call it a ‘dwarf planet’ instead. Since this decision was not actually binding, many people (including scientists) still consider Pluto a planet. If Pluto isn’t actually a planet, then Neptune is the planet with the longest year (164.79 Earth years).