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
15810 Arawn resonance with Neptune, similarly to dwarf planet Pluto, the largest known plutino. It measures approximately in diameter, based on an absolute magnitude of 7.6, and estimated albedo of 0.1. In April 2016, a rotation period of 5.47 hours was determined for this minor planet. In 2012, Arawn was hypothesized to be in a quasi-satellite loop around Pluto, as part of a recurring pattern, becoming a Plutonian quasi-satellite every 2 Myr and remaining in that phase for nearly 350,000 years. Measurements made by the "New Horizons" probe in 2015 made it possible to calculate the motion of Arawn much more accurately.