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Why is Neptune the outermost planet from the Sun?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

Neptune is considered the eighth and outermost planet of the solar system because it is the farthest of the eight major planets from the Sun.1

While Pluto was historically considered the ninth planet, it was reclassified as a dwarf planet in 2006 by the International Astronomical Union (IAU). This decision was based on a new, rigorous definition of a planet, which requires a celestial body to be large enough to clear its orbital neighborhood of other objects. Because Pluto is an icy, low-density body that shares its region with other Kuiper belt objects, it does not meet these criteria.234

Regarding its current position, scientific models—such as the Nice model—suggest that the giant planets, including Neptune, formed closer to the Sun and migrated outward to their present locations due to gravitational interactions with leftover planetesimals.5