Considering the nature of the game, you may want to take over a planet. In fact, one of the main objectives of the game is, obviously, to rule the universe. Thus, taking over planets is actually a very important part of the game. However; if you're taking over an owned planet, you're probably going to have to fight for it. Likewise, if you own a planet, it is likely someone may try to launch a military invasion on it. This is a guide to taking over planets. (This guide also works for taking over ships.)
1. Pick A Planet
Somewhat obvious, you need to figure out what you're taking over first.
2. Make A Battle Plan
Obvious as well. Work out the questions. How are you going to attack? How big will your attacking force be? Are you going to attack by ground or space? With magic or ships?
3. Penetrate Defenses
If there are any immediate defenses (protecting spells, laser beam force fields, etc), you're going to want to get past those.
4. Take The Planet
Take over capital buildings, place your flag on the planet, remove symbols of the old government.
5. Wait Three Days
If the enemy does not respond in three days, the planet is automatically yours.
6. Defeat any contesting forces
Defeat the enemy.