
If on a new planet the amount of fields for building will not be enough, you can leave the planet and re-colonize it in order to choose the place for a base with a different amount of fields. Astrophysicist can help.
When colonizing an occupied object, you must win a battle with its owner. All defenses and fleets of the old owner are destroyed, and all buildings except those not destroyed lose 1 level (on asteroids buildings are completely destroyed).