You'll be worried a lot more about making the ability than the planet
Making it is easy. Each planet type that exists has a corresponding .entity file in the GameInfo folder (PlanetTerran, PlanetIce, PlanetDesert, etc). To make a new one, you just need to copy one of the existing files and modify it however you like. Each entity is allowed 4 abilities, and planets are no exception:
ability:0 ""
ability:1 ""
ability:2 ""
ability:3 ""
They don't have any by default, but can be given some. So let's say you make your ability and it's called "AbilitySpawnCapitalsOnColonize.entity" (to be descriptive and use same naming convention), to make the special Terran planet that uses it you can just copy PlanetTerran.entity, rename it something like PlanetTerranSpecial.entity and attach the ability:
ability:0 "AbilitySpawnCapitalsOnColonize"
ability:1 ""
ability:2 ""
ability:3 ""
You could then use PlanetTerranSpecial in the templates for map making and insert it into your maps. It will need the mod running to function properly, of course. To learn more about how modding works and to get a little practice, check out the sins_modset.pdf in the Forge Tools download (site's downloads section). You'll also need to download them to work with the entity files - the game ones are in binary so you can't edit them, the forge tools give you text versions.
Hope this helps a bit 