In the game Endless Space the map the Icon represent a solar system. It has a sun, planet and astorides wjem you enter the system.
While in Solar Empire the Icon in the map is usually one planet with it resourse rocks.
Hopefully I'm explain it better...
You would need to make a mod to do exactly that. New models where the planet model is replaced with one mesh containing all the planets, moons and the sun of the system. Then every map would have the changed looks. The names and description of the planets would also have to be changed (terran planet to sol system for example) to match the meshes.
It would look odd though when the system is lost by bombarding the sun, and other strange thing might ensue.