You need to add a copy of your video (so you have two layers, only the bottom one needs the planet on it). Rotoscope the top layer - when you create your mask the only thing you will see is the bit of video inside the mask, the blade in this case and it will sit above the planet because it is the top layer. If you use more than one mask you will need to set ...
If the blades are rotating, and the camera is moving, you need to create a good foreground layer with anything the planet is supposed to be obscured by.
So from the image it looks like it will be obscured by the spinning rotors.
You need to find a good way to make a layer containing only the rotors.
You could make a mask and move it frame by frame, but if ...
You can replace a comp in the timeline with a comp from your project tab:
Select a comp in the project tab
Select the layer in your timeline that you want to replace
Hold down alt
Drag and drop the new comp from the project window
It will replace the layer in the timeline, maintaining all the settings on that layer.
You can show or hide a layer using the ...
If performance is sluggish, I would make sure the videos are the same resolution and frame rate as the format you plan to deliver your final video in.
So if the videos you are trying to import are 4K, but you're working in HD in After Effects, I would transcode the videos down to HD.
Also use a non-GOP codec like ProRes instead of h264/mp4. They perform ...