- (archaic, astronomy) A moon of a planet besides Earth; a secondary planet.
1723, John Harris, “Planets”, in Lexicon Technicum: Or, An Universal English Dictionary Of Arts And Sciences:
- The Motions of the ſix Primary Planets round the Sun is So adjuſted, that the Square of the Times of their peroidical [sic] Revolutions are as the Cubes of their Diſtances from the Sun : And the ſame thing is found by all Aſtronomers to be true, with Regard to the Motions of the Secondary Planets or Satellites round their primary ones. [...] But if the Orbit of the Satellite Planet, inſtead of being a Circle be an Ellipſis, in whoſe Focus the Primary Planet is ſuppoſed to be placed, then will the greater Axis of this Elliptical Orbit, twice advance forward, viz. in the 2 Quadratures, and twice recede backward, viz. in the 2 Syzygies, of every Revolution of the Satellite round the Planet.
- (astronomy) A planet viewed as a satellite of another object, such as another star; an exoplanet.
1970 January 17, Stirling Colgate, “Ejection of Companion Objects by Supernovae”, Nature, volume 225, Letters, pages 247–248:
- It is possible that supernovae occur in conjunction with either a satellite planet or binary star.
- (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.
ca. 2009, Challenger Center, “Satellite Planets with Dr. Alan Stern”:
- it's only been in recent years that astronomers and planetary scientists have been willing to think of planets, like they've long thought of stars and galaxies, as capable of orbiting one another. And of course those objects are the satellite planets.
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.