Okay, here are my thoughts. The planet looks very clear, perhaps too clear. I know space doesn't have any atmospheric depth, but blurring it slightly might help it blend in. Especially the edges; I think JasonN blurred the edges when he did this for his edit. As it is now, the planet (especially the border) is very distinct and doesn't seem to be part of the original image. The scene is bright overall, at least how you exported it, so that might be what makes it look so noticeable. I can't tell whether the brightness should be changed for the clip or the new element, but something doesn't quite fit brightness-wise between the two.

This is a minor point, so please ignore it unless it's a very easy change: The planet seems very big. It could stand to be a little smaller, which might help it blend into the rest of the picture anyway. If you have the motion tracked and all that, I wonder if it'd be an easy thing to change, but like I said it doesn't really matter much.

I am totally unfamiliar with rotoscoping and image manipulation, so I could be talking out of my ass. If it is possible, could you export it (or screencaps) with its original brightness? Maybe it's my Mac struggling to play a wmv file, but it looks too bright and that might be skewing my judgement.