Planets have climates. The climate of a world is determined by its temperature in °W (Wisseman).
Temperature Range Climate 0-14°W Arctic 15-39°W Cool 40-60°W Warm 61-84°W Tropical 85-100°W Desert
While temperatures below 0°W and above 100°W are, of course, possible, they are so inhospitable that even extreme measures aren't adequate to create an environment capable of supporting even 1 clan for a prolonged period of time.
Most races do best on planets with a temperature of 50°W. The Crystal and Siliconoid races do well on warmer planets with a temperature of up to 100°W.
The Colonies, Fascists, Robots and Fascists are able to withstand desert climates well enough to support a small desert outpost of 6,000 colonists on a planets with a temperature of 100°W. The Rebels are able to build outposts that support up to 9,000,000 colonists on ice planets (a temperature of 19°W or less). All other races can maintain only very small populations on an extreme desert or arctic planet.
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. 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.
It is possible to permanently modify the climate of a planet by the use of a Terraforming Ship.
An excellent offsite population calculator can be found here.