Once thought to be a planet, Pluto is now recognized as a dwarf planet due to its mass and inability to sweep its neighborhood clean. While most objects in the solar system orbit the sun on essentially the same plane, Pluto's orbit takes it outside of the traditional range. These significant differences mean that the dwarf planet formed a little bit differently from the larger planets in the solar system.
The early solar system
Approximately 4.6 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the center of the nebula.
With the formation of the sun, the remaining material began to clump up. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind swept away lighter elements, such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create smaller terrestrial worlds. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants. In this way, asteroids, comets, planets, and moons were created.
Pluto's rocky core would have been the first to form under this scenario. From there, gases and ices would have been pulled in gravitationally, with the core growing larger and larger. But like the other dwarf planets, Pluto failed to gather enough mass to reach the classification as a full-scale planet.
Rejected by Neptune?
The early solar system was a cluttered place, with planets, dwarf planets, and asteroids scattered all around. As planets traveled through their paths, their significant mass gave them a great deal of gravitational power, changing the orbits of smaller bodies. Some were catapulted into the sun, while others were cast out of the solar system completely. The solar system today is clean compared to its early life.
Pluto's bizarre orbit led many astronomers to conclude that it had been kicked from the orbit of the planet Neptune at some time in its history. This seemed strengthened by the planet's similarity to Neptune's largest moon, Triton. However, Pluto's orbit is such that it can never collide with the large planet, meaning that it could not have once been part of its system.
Part of a larger group
Once scientists demonstrated that Pluto could not have been a part of Neptune's system, they began to search for another reason for the then-planet's bizarre behavior. Their search led them to the discovery of the Kuiper Belt, a cloud of rock and ice on the distant edge of the solar system.
As a planet, Pluto stands out in terms of temperature, distance, and orbit, but as a Kuiper Belt Object, Pluto is but one of millions of chunks of rock and ice. Admittedly, Pluto is the largest known object in the region, but the dwarf planet Eris falls as a close second, and was originally thought to be larger than Pluto. Indeed, the discovery of the massive dwarf planet in 2005 led to the reclassification of Pluto as a dwarf planet rather than a full-scale planet in the solar system.
— Nola Taylor Redd, SPACE.com Contributor