I have a bit of a problem with the nuke explosions. A nuclear explosion isn't going to go that high. Sure it's a couple of miles high, but when you consider how big the planets are, it should only be a bit of a flash followed by and glowing spherical (viewed from above) cloud, not a mushroom cloud bigger than the atmosphere itself. For frame of reference, watch the Battlestar Galactica miniseries, or the end of Terminator 3, those have some pretty realistic looking planet side nuclear explosions.
I agree. The atmosphere compare to Earth is like paper compared to a basket ball. The atmosphere is several miles high (more than 10, I think) and a mushroom cloud is (I think) only two or three miles high. Plus, how do we
know what a nuke going off from space looks like, there has never been a picture of one.
If the nukes where really that big, then there would also be more radiation. That would kill the popultion faster and you wouldn't be able to colonize it for years and years. It really does take awhile for everything on the planet to be gone, espescially with super-de-duper nukes.