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 list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets."" As of Feb 2018, Brown's list includes 952 objects, identifying ten known trans-Neptunian objects—the four accepted by the IAU plus the six bodies: , , , , and —as "near certain", with another 26 "highly likely". Notably, (1535 km) has a larger diameter than the dwarf planet Makemake (1430 km). Stern states that there are more than a dozen known dwarf planets. Only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The