Populating my scene

I began populating my scene with planets after I began my animation, I did this because I only wanted the planets to be in a shot, instead of just sitting in the background where they may not be seen and increasing the render time unnecessarily. What I mean by this is after I had my camera set up and had finished it filming what I wanted it to I would add a planet after to fill the scene and make it look better, allowing me to only have planets that would definitely improve the look of my shot.