If a planet has more colonists than its climate can support, it is considered overpopulated, and a percentage of the colonists may die each turn.
Normally, on planets with a temperature below 15 or above 84 degrees, the number of colonists that can survive without assistance is limited. The precise number depends on the precise temperature, the climate death rate (default is 10%), and the colonist race. In extreme circumstances, it's possible for a planet to be overpopulated with even a single colonist, and all planetoids are considered to be that extreme unless they have a mining station in orbit.
Colonists in excess of the maximum may survive by eating supplies. One supply unit is consumed per 400 excess clans per turn (truncated, plus one more for simply being overpopulated), and four supplies must be present per excess clan ( after production and consumption) to avoid losing population.
Death due to overpopulation may result in a planet having more structures than the population can support. When there are not enough colonists on the planet to maintain structures on the planets surface the structures will decay at a rate of 3 structures per turn. This applies to mines , factories and defense posts.
For detailed formulae relating to overpopulation, please refer to the main population details page.
For a handy offsite reference, try the Planets Calculator.