What do you mean a new planet type? Theres only 3 atm, Dead, Habitable, Gas Giant. if you mean the ones with bonuses, they are Planet Traits, PlanetTraitDefs.xml
Yes, I created some new traits (Utopian World, Ancient World, Lost World, etc) and assigned them to planets in the SOL system using StarSystemDefs.xml. Those planets just came up generic though. I did cut and paste from Serene World, Desert World, etc to make sure I had all the fields. But, those SOL planets never took on the traits. They will only take on the canned traits (Serene, Active Core, Toxic, etc).
Just trying to see if there is somewhere I missed to get those new traits added,
