Aurora is on version 2.5.0 C#, available at the Aurora Forums.
Contact Erik on the forum for a wiki account.
Colony cost
The Colony Cost of the planet is a measure of a planet's (or moon's or asteroid's) suitability for sustaining human (or alien) life. The higher the Colony Cost, the more effort it takes to build habitable cities there.
If a planet has zero colony cost, like your homeworld, the population is not restricted in any way other than the maximum growth rate. Otherwise, the maximum population is equal to (Infrastructure * 10,000)/Colony Cost. For more details on colony cost, see the System View window (F9).
The Colony Cost of the planet can be lowered by terraforming.
Note that the civilian sector produces and ships infrastructure to your colonies by itself, although perhaps not in great quantities. See Trade System for details.