Planets are massive chunks of matter from a star. They serve as different maps in Gun Bros.
Cerberus Prime: Filled with lava and volcanoes, it is the first planet unlocked. Enemies are very basic, especially if you have the skills to lure them around the explosive barrels. The barrels can do MASSIVE damage, with a good damage radius, but they are ineffective against enemies in the higher revolutions. The main enemy that will cause trouble is the Tank Head. It is completely invulnerable, except at its back, and it will fire rapidly in sets of 3.
Haven: Haven is a more "modern" Cerberus, a small town. There are many walls in this planet. There is a Spire in the center of the planet. When you touch it, all enemies slow down for about ten seconds. The spire recharges after two or three minutes.
Bokor: This planet doesn't have revolutions, instead, it has hordes, and by passing each the enemies get tougher and tougher. If you can avoid getting hit by zombies for a long time, you will rack up a large killstreak, resulting in a good score. The enemies are pretty much the same - zombies. Most are killed instantly, but a few have large amounts of health.
Yeroc Sina: This is a jungle planet with "organic" enemies. For example, instead of the man-made Pus Tank, there is a chain-gun-wielding Wild Bill. Like Cerebrus, the planet is filled with bombs that can be detonated once to destroy enemies. Every minute or so, a "babe" appears on the map, and a HELP message shows where the babe is. Standing near the babe revives her, and she will fight on your side with a powerful weapon for about thirty seconds. Unfortunately, there are many enemies on this planet that shoot projectiles, this makes it a hassle to get perfect waves. Also, many enemies will not yield as much XP as their Cerberus counterparts.
Ceres 2: Ceres 2 is the more "Elite" version of Cerberus. Every enemy will give twice as much XP as in Cerberus, making it a good XP farm. However, every enemy will have more health and deal more damage than their Cerberus counterparts. There are two turrets in the corners of the map. When you touch them, they rapid-fire devastating energy blasts for about 10 seconds. Each one must recharge for two or three minutes, and then the other one activates.