Natural satellite been directly observed, have been called "moonlets". Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets. The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger
Dwarf planet has itself been somewhat controversial, as it could imply that these bodies are planets, much as dwarf stars are stars. This is the conception of the Solar System that Stern promoted when he coined the phrase. The older word "planetoid" ("having the form of a planet") has no such connotation, and is also used by astronomers for bodies that fit the IAU definition. Brown states that "planetoid" is "a perfectly good word" that has been used for these bodies for years, and that the use of the term "dwarf planet" for a non-planet is "dumb", but that it was motivated