Double planet
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In astronomy, a double planet is a binary satellite system where both objects are planets, or planetary-mass objects, and whose 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).
Artist's impression of the Eris-Dysnomia system
Satellite system (astronomy)
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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, although
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.
Model for formation of Jupiter's rings
Gas toruses in the Jovian system generated by Io (green) and Europa (blue)