I did it a long time ago, but I am having trouble now (Entrenchment v1.01 so you know). The Developer tells me:
GalaxyScenarioDef Error: Invalid EntityDefName(PlanetSpacePony) found for 'SpacePony'.
...when loading mod, yes I am adding a planet Space Pony, for a friend and I, we love to harbor candy mountain refugees...
and:
GalaxyScenario type name is not valid. The EntityDefName is not available. TypeName:'SpacePony' -> EntityDefName:'PlanetSpacePony'.
...when loading a map with the planet type in it. I have added the orbit body type, and the planet type (and the strings) in the galaxy scenario def file. The file is called PlanetSpacePony.entity, and so I am wondering where else I need to include references to it.