I came up with the idea of convict planets so that I could scratch the Trigun-style space western itch in Assholes in Space without having to seriously redefine the game world. One of the things that I really appreciate about science fiction settings in general is that a completely new environment or set of social rules can be just a planet away, like how Firefly and Cowboy Bebop can switch from gleaming future cities to frontier satellites in between episodes. Plus, the idea of desert planets conquered with jury-rigged technology just tends to appeal to me.
Criminals being sent to a convict planet are generally launched from Federal prison transports in dropships known as Oneways. Each Oneway is only equipped with stabilizing thrusters and enough fuel to move the ship into an atmospheric entry, where an ample amount of heat shielding and impact-resistant foam should protect every passenger in the crash landing. During the drop, a recorded message will inform the prisoners about the basics of their new home, its environmental hazards and the existing power structures (if any). Once they crash and extricate themselves from the foam-filled mess that was their transport, they're on their own.
Some convict planets have developed far better than their overseers anticipated, creating towns and cities from both the planet's natural resources and the salvaged wrecks of their Oneways. Generally, these more developed planets are either controlled by feudal warlords or else divided into civilized outposts and roving gangs of raiders and thieves. Towns are simultaneously wary and welcoming of new arrivals, like a constant Yojimbo-style struggle for power. Of course, there are plenty of other convict planets that have turned out more like the Most Dangerous Game.
Picture from Greg Martin's super cool guide on how to make planets in Photoshop, which I will totally start doing once I have some free time.