Hitting tab again will take you back to the planet view of the planet you were last on. Unless you want to have a shortcut for specific planets. I don't think there's any way to implement that for a game that could have 120 planets in it. However, maybe a hotkey to go to your homeworld, or go to the most recent planet that is under attack, or jump to any planet that is under alert for incoming waves. Those could probably be mildly useful.