When a ship colonizes a planet using the Colonize mission, the planet will start with the former experience level of that ship.
When you capture a planet in combat, that planet's experience will be reduced according to the effective colonist survival rate, see Planet Combat Effects formulas. With the default configuration, the planet will start out with no experience.
Planets you colonize normally, by dropping clans, ground combat or Imperial Assault, start with zero experience.
