Colony Ships are the only way, barring conquering enemy colonies, of adding new planets to your empire.
They can only be built by and at your colonies, and aside from the basic ship components, they need a Basic, Enhanced, or Massive Colonization Module.
It should come as no surprise that the gigantic Colonization Modules mean that Colony Ships do not need to obey the size restrictions imposed by your Construction Yard research. They're also slow and expensive.