In astronomy, a double planet is a binary satellite system where both objects are planets, or planetary-mass objects, and whose joint barycenter is external to both planetary bodies.
Visual comparison of the sizes of Earth and the Moon (above right) and Pluto–Charon (below right)
The Pluto–Charon system is closer to binary than the Earth–Moon system (distance not to scale).
The Earth–Moon system is sometimes informally referred to as a double planet (masses are roughly proportional to volumes, not surface area).
Artists impression of the Eris-Dysnomia system
Satellite system (astronomy)
A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.
Saturn, its rings and major icy moons—from Mimas to Rhea.
Formation of Pluto's moons. 1: a Kuiper belt object nears Pluto; 2: the KBO impacts Pluto; 3: a dust ring forms around Pluto; 4: the debris aggregates to form Charon; 5: Pluto and Charon relax into spherical bodies.
The Pluto-Charon system (with orbital paths illustrated): The binaries Pluto and Charon orbited by Nix, Hydra, Kerberos, and Styx, taken by the Hubble Space Telescope in July 2012
Model for formation of Jupiter's rings