You can in increase the Amount of Habitable Planets
Edit your MapSetupDef.xml. Under the Abundant map settings section increase the percentage:
MapSetupDef.xml is located in C:\xxx\Steam\steamapps\common\Galactic Civilizations III\data
It's been a long time since I played, hope that helps.
Edit: For Crusade it's located in C:\xxx\Steam\steamapps\common\Galactic Civilizations III\DLC\EXP2_Crusade\Game