I think that in the beginning of the game, earth will have the best stuff. Every other planet you CAN travel to will be way to difficult and enemy-filled to explore. Then you get enough stuff yo survive while exploring the moon and you find better stuff there. Then you find enough stuff to explore mars and you get even better stuff there and so on. And when you go back to planets, they will be planets you already "conquered" but u find an enemy there u didnt know was there before, you go after them and it's a pretty even match despite your gear. Then you get the same level reward as you got as the current planet your in the process of "conquering".