These weapons are short range weapons that only do damage to Defensive Structures. They do not attack ships. Although you could use a fleet equipped with the other four types of weapons to destroy defensive structures, you would use a lot more He3 to accomplish this and it would take you much more firepower to do so. Planetary Weapons are designed to specifically attack these defensive structures quickly while using as much He3 per round as Ballistic Weapons. With a cooldown of 1, they can attack these structures every other round. Their range is 1-2 squares so you will need a ship high in movement to engage quickly and to be able to keep up. Since these type of fleets will be attacking the defensive structures, you probably will want to load up on some shield modules and structure modules as well.
Planetary Weapons Stats
The following table is a list of all Planetary Weapons, their base attributes and the base cost per module installed on ships. These weapons are only effective against defensive structures.