planet, says the committee headed by Harvard astronomer Owen Gingerich, is an object that orbits a star like the sun; is massive enough so its own gravity keeps it roughly round; and isn't a satellite of another planet, like Earth's moon.
That definition already confounds many astronomers: Charon, proposed for promotion to a planet, orbits the sun and is round, but most astronomers have long held that it also orbits Pluto, which makes it a satellite.
As for Charon, the committee of astronomers decided that it isn't really a moon because Charon and Pluto circle each other around a common center of gravity -- making the pair, in effect, a "double planet."
Q: What is a dwarf planet?
A: A dwarf planet is a term generally used to describe any planet that is smaller than Mercury. Note that the term “dwarf planet” is simply a descriptive category and not an IAU definition. Terms such as “terrestrial planets” and “giant planets” are additional examples of descriptive categories that are not IAU definitions.
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