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
Atmosphere of Pluto molecules of nitrogen and 5×10 molecules of methane every second. This presumes a loss of several centimeters of nitrogen ice and several dozen meters of methane ice during the lifetime of the Solar System. Molecules with high enough velocity, which escape into outer space, are ionized by solar ultraviolet radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted, possibly forming a shock wave upstream of Pluto. The ions are "picked up" by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail. The