Well, I found something strange in your model: The axis of the JointSU component was not in the center of the component. I do not think you can precisely control things with that, despite the result in your model was interesting. To propose a planet system model, I changed the axis of your jointSU back on center.
EDIT: The JointSU component in fact is just its axis (and the name). The geometry we create around this axis is just anything to make easy to position this axis in it's parent component. I just created new geometry around the axis of your JointSU and deleted the old one that was far away from the axis. See how your joints were working.
atom1.skp
Obs: I prefer to name joints with numbers instead letters. But its just preference.
Each Planet_System (child of Sun) will have one joint(1) to be aligned with the Sun joint(A).
This Planet_System joint(1) will control the rotation speed around the Sun (year rotation). So, you need to animate this Planet_System joint(1).
Each Planet_Ball (child of Planet_System) will have one joint(1) to be aligned with the Planet_System joint(2).
This Planet_Ball joint(1) will control the rotation of the Planet arount it's axis (day rotation). So, you need to animate this Planet_Ball joint(1).
And so on ...
I let the model very incomplete so you can have fun modeling the solar system, by the way an interesting training. You can also model the Moon around Earth, etc ... nice.
We can not animate the translation of the joint for now, but that is not difficult at this point. With that it's possible to establish a elliptical orbit instead a circular one ... as soon I find some more time.
I put some stuff on hide in order to make a nicer animation. Just start animation ...
atom2.skp
Regards
Marcio
how many Joint-children can be allocated to a parent?
No limitation, but there is no check for an hiper-static condition, for example:
Parent has joint(1) and joint(2).
Child has joint(1) and joint(2).
Establish joint connections (1)-(1) and (2)-(2). This is an hiper-static condition and I do not know what will happen if we ask (1)-(1) to rotate in one direction and (2)-(2) in another ... but the idea is not complicate too much. It's already enough complicated.
PS: Found more errors on the code, so ... new update.
JointSU.rb
JointSU_GUI.rb