I am currently completing a project for school (due Wednesday) involving the Sun, Earth, and Moon (Moon revolving around Earth; tides shown).
In the animation the camera will show the Sun, then pan to show the Earth and zoom out (the tidal movements will be apparent); however, after placing a Sun project (already created) in a pre-composition, I attempted to place a 16k galaxy background behind the Sun. In order to allow the camera to pan over a large space, I tried to reduce the scale of every object within the pre-composition, however, the section of the background shown in the provided video was reduced (entire background was not preserved). I then moved the background below the entire pre-composition, but it would not appear, as the pre-comp did not have a transparent background.
How may I include the Earth, Moon, and Sun in fixed locations on a galaxy background (link to project below)?
I haven't got time to open your project and take a look but it sounds to me like you don't have a very good understanding of AE's 3D space and what happens when you pre-compose 3D layers and turn on the collapse transformations switch. If it were me the star background would be a 2D solid that just had a position animation to match the camera move. Much easier than making the background 3D.
If I get a chance later tonight to look at your AEP file I will be able to give you some more suggestions. In the mean time I suggest that you start by making sure the layers in the pre-comps are all 3D and that you have Collapse Transformations activated.