Creating the solar system is shorter and simpler but still harder, it involves dealing with both the entity and the visual tree.

Create a script to initialize all the planets, put it under the scripts directory (nebula/data/scripts), add loadscript scriptname.tcl to the end of go.tcl

You then need to create an entity to represent a planet. Go to /lib and copy one of the existing directories (asteroid would be good) to make a planet directory.

Go into that directory, and change all the references to asteroid to planet.

Copy the appropriate meshes/textures into planet/data.
You can find gorgeous textures for most of the planets online, many of them are already included in nebula/data/lib/tex/planets.
You can use one of the sphere meshes under lib/meshes.

In your script, add some gen_ent planet to create planets.

use sv pos x y z to place them in various places.

Next is gravity.

First of all model it as a single parent-child relationship, where each planet is attracted to one other. You can use reference variables to do this very easily. As a bonus you can do n^2 attractions, either using a server to manage all the information or searching out other planets in the tree (second one is easier).

I created the class already, but it is nearly empty, the one thing i did do is set up the variables for you since it is kind of weird. Figure out how to use position, and attractedToPosition to apply the force (masses too).

I wil finish the rest of this soon, that should be enough for a few days.