Went do the download page here:
Downloaded Galaxy Forge.
Only the most basic planets are available to choose from. Asteroid, volcanic, ice, terran, desert.
There must be a GalaxyScenarioDef.galaxyScenarioDef file somewhere that updates this. I know there is, because in the past I have been able to build maps with urban moons, savannah planets, etc.
I mainly run the Distant Stars mod.
Anybody have the info?