The inner planets were formed of rock and metals. The gas giants initially formed as icy planetesimals, then as they grew, their increased gravity attracted hydrogen and helium gases. This is known as the model of Gas Planet Accretion. Pluto didn't form in either of these ways. It is ice and rock, so it is more like a comet. There are many objects like Pluto, so Pluto had to be reclassified as a dwarf planet.