What are all the differences between a capital planet and a normal planet?

My understanding is that it cannot be destroyed and is always visible in your profile but I'm pretty new at this.

Capital cannot be lexxed, nor can it be destroyed by most "normal" means.  Solar destruction (nagibator) still spells doom for it tho.

Also, when you get the Toss attack (Phoenix) he does not attack you capital planet.

And I think that's pretty much it 

capital holds your "key" artifact. Upon loss you pay resource tribute to superior player, and vice versa. Capital planet is the only one that possesses the key.