(plural satellite planets)
- (archaic, astronomy) A moon of a planet besides Earth; a secondary planet.
- (astronomy) A planet viewed as a satellite of another object, such as another star; an exoplanet.
- (astronomy) A moon of planetary mass; a natural satellite which would be considered a planet or dwarf planet if it were in direct orbit of the Sun.
In archaic usage: the phrase satellite planet was used for a few centuries after the Galilean and other moons were discovered, before the word "moon" was extended in use from Earth's moon to the satellites of other planets. At the time, all bodies of the Solar system beside the Sun were called "planets", and the phrase satellite or secondary planet was used to distinguish satellites from the primary planets they orbited.
In modern usage, the term is used by astronomers who define a planet strictly by its physical characteristics. There are 19 at least satellite planets (planetary-mass moons) in the Solar system, including: the Earth's Moon, the four Galilean moons, Titan and the half dozen medium-sized moons of Saturn, the five medium-sized moons of Uranus, Triton, and Charon.