Bombs are a special weapon that can only be used to attack planets. They tend to do much higher damage than other weapons of the same space, but to balance that they cannot be fired at enemy ships.
Planets get increased shielding ability to account for this additional damage. Class V, X, XV, and XX Planetary Shields can be researched and built (at each individual world) which add 5, 10, 15, or 20 damage to the shield level of the missile bases present on the planet.
Bombs and Missiles do full listed damage to planetary defenses, all other weapons do half damage.
Missile bases can be constructed on planets as defenses. These have regular shields and hit points like ships but they are on the same space as the planet and they cannot move off that space. Each missile base is equipped with 3 of the best missiles available when it is built and it has unlimited ammo. They can be upgraded as newer better missiles, targeting systems, and shields are developed.
Note: Generally, the nature of missile bases is that they are very very hard to kill if the opponent has researched and implemented planetary shielding about equal to their stage of the game. Missile bases benefit from both the highest regular shielding available and the planetary shielding AND they benefit from taking half damage from most weapons. At the max shielding level, Class XV regular shields and Class XX Planetary shields there are very few weapons that will do any damage at all to the missile bases.
If the opponents cannot destroy the planetary missile bases they must retreat and come back later with a force that can defeat them. Even 1 missile base, with an appropriately advanced shield, will turn away entire enemy fleets if they are low enough tech. They may not kill them, but the enemies will be forced to retreat after combat.
There is a special kind of weapon, a biological weapon that does not do damage and instead directly kills planetary colonists. These weapons can kill 1 (Death Spores), 2 (Doom Virus), or 3 (Bio Terminator) people per weapon per shot and they will bypass missile bases completely. They are capable of completely destroying all the people on a planet, and by extension the entire planet, without harming any missile bases in the process. If the planet is reduced to 0 population, the missile bases and planetary shield will be destroyed through years of lack of maintenance and they will have to be re-constructed if a civilization tries to re-colonize the planet at a later time.
There is a special defense called the Bio Toxin Antidote (1) and the Universal Antidote (2) that saves a number of people equal to its ability from each biological attack. When the Bio Toxin Antidote is researched, Death Spores are useless, Doom Viruses kill 1 per weapon per hit, and the Bio Terminator kills 2 per weapon per hit. With the Universal Antidote researched both Death Spores and the Doom Virus are worthless and the Bio Terminator only does 1 per hit per weapon.