Composite interior and exterior separately I'm trying to make an interior scene where I want to composite the exterior separetely form the interior. So, before I go any further I want to set up the layers to accomplish this.

In Layer 1 I've got just the interior:



Layer 2, the Sun and the Earth without the atmosphere:



Layer 3, just the Atmosphere, which I need to blur.



How can I composite the interior and the exterior separately and the join them?
 I'm fairly new to compositing so I don't have a clear idea to do this, specially with the window in the way.



EDIT:

This is how I've set up the three different render layers:

Layer 1:



Layer 2:



Layer 3:



And this is the result using the Alpha Overlay in the Compositer:



Thanks,
With the help of Cegaton I've found the solution. The main problem was that the window is in the way so it has to be rendered separately. This way there's a hole in the first layer without the background so later on the Alpha Over node can be used.

So, in this case, four render layers are necessary:

Layer 1:

In this layer is the foreground, just the spaceship but without the Window, masking the rest so that there's a transparent hole in the window.



Layer 2:

In this layer it's the Earth without the Atmosphere.



Layer 3:

Just the atmosphere in layer three.



Layer 4:

Here is the Window:





And finally, here's the compositor node setup: