why not? We can already make parts, and all that is is providing a model, and then filling out a set of parameters for said model and you got yourself a part planets shouldn't be that much different, just provide the size, and a bumpmap and then fill out some parameters, such as gravity atmosphere, distance from kerbol, apoapsis+periapsis, etc.. if an API for parts came out relatively quickly in the scope of KSP's development, I envision that an API that would create planetary bodies or moons should be in the works very soon.