It depends on what you are focussing on. Do you plan to write more related, XPlanet modules, or would you like to see more XPlanet modules? Then you should use something in the XPlanet:: hierarchy.

Or is the API the most important, and it just happens to be that it's implemented wtih XPlanet? That is, you want to write (or see others write) more modules with the same API with a different underlaying machinery? Then something like Foo::Bar::Xplanet works well, for suitable Foo::Bar

-- Abigail