I don't seem to realize the way of attacking other player and the damage that they get, if you continued to attack other player's planet nothing would happen to the planet except you get some of the resources,nothing happen like you destroy his planet that he have to get a new one and a new base it seems that he is immortal, every time you get resources only!!!

Only fleet and defenses can be destroyed. And the moon too.
That's the way this game goes.

is it possible to completely wipe out another player leaving their planet completely empty, and therefor possible for me to take as my own.

No.