Practical DFT is nearly always done within the framework established by Kohn and Sham in 1965. We’ll have a look at their method:
DFT is a method to describe a many-body system: a system of more than two mutually interacting particles. Atoms, molecules and solids are many body systems based on the electromagnetic interaction. The solar system is a many body system based on gravitational interaction. Describe in words how you would use DFT and the Kohn-Sham method to study the many body system of gravitationally interacting planets and the Sun.