Expression request

There's another way to create a very 3D starfield all inside AE that is very convincing.  You only need 2 layers and some expressions. The trick is offset, blend modes and scale. If your starfield has an alpha channel you don't need to use blend modes.

Here's the theory. You create a starfield similar to the one you have that is about 4 X the comp size, let's say 4000 X 4000 pixels. If the star field does not contain very many unique features then it doesn't need to be much bigger than the comp. The trick to the star field is that it needs to be setup to be usable as a tile. IOW, if you were to offset the starfield 50% you would not see a seam. You create another star field that's slightly different. There are a bunch of ways to create this kind of star field inside AE with CC Ball Action or with Photoshop using nothing but noise and threshold. 

Here's the real trick, the starfield layers are 2D, they just look 3D because of the difference in scale and offset.

Now you tie the scale of both star fields to the distance from the camera using an expression so that they both scale at a ratio of the distance between the camera or the camera dolly (I almost always use a null to move and point the camera. Just as in the real world, a dolly or null, gives you more precice control).

You then tie the angle of the camera's orientation to the value of the offset. As the camera pans and tilts the background is offset. Each starfield layer has a slightly different value so they move at slightly different speeds.This is why you must have a starfield image that's seamless. As the camera moves in any direction you'll get parallax between the stars that makes them look 3D. It's basically the way they made starfields in Star Wars.

I made a starfield in the 70's by simply poking holes in a piece of black paper, putting a light behind it, then dollying into the paper slowly. I rewound the film in the camera, turned the paper upside down, then photographed similar movement a second time double exposing the film. This gave me a 3D effect of moving through space that was very convincing.

I set up this comp in just a few minutes. There's a shape layer representing a plannet. I put in a camera move with a tilt and pan so you can see the 3D effect. There are a couple of sliders to adjust the relative offset for tilt and pan. Rotation even works (rotate in Z). Simple, convincing, and with the right star field layers, beautiful. There's no complex math. Everything came right from AE's built in expression library.

Enjoy.