How do temperature differences in the protoplanetary disk explain the arrangement of the planets in the solar system?
Temperatures in the protoplanetary disk influence the type of planet formed.
The inner area of a protoplanetary disk is closer to the sun and is hotter. This means that volatile substances are gases. The gases tend to get moved to the outer regions by the solar wind.
This is why the inner planets are rocky.
Further from the sun, the volatile substances become solid and gases get trapped in ice. This allows the creation of gas and ice giant planets.
In fact the planets are not in their original positions. Once formed the planets move positions due to gravity. It is thought that Jupiter migrated towards the sun and then moved away again and changed the setup of the solar system.