Population
The population of a world is necessary to work its mines, run its industry, manage its planetary defense--and feel a sense of national pride. You cannot own a world that has 0 population.
Normally, a world's population will grow by about 10% every turn, up to its population limit, and then stop growing. The population on an unowned world will not grow; neither will the population on a world that is still recovering from a recent plunder. A robotic population will not grow by itself, but must be built. If the world is owned by an Missionary, then all of the new population each turn will be converts.