Pluto used to be a planet. However, in 2006, after the discovery of the trans-Neptunian object Eris, which apparently has a greater mass than Pluto, the International Astronomical Union created the intermediate designation of "dwarf planet" for objects orbiting the sun that are sufficiently massive that gravity forces them into a rounded shape but have not "cleared the neighborhood of its orbit". This definition was more or less tailored specifically to prevent Ceres from being promoted to "planet" status, and in the process it demoted Pluto (and Eris) to "dwarf planet" status. The distinction between planets and dwarf planets is fairly artificial, among other things, the "cleared the neighborhood of its orbit" portion of the is fairly arbitrarily defined (and the mere existence of trans-Neptunian objects pretty much means that if fairly applied, this provision would eliminate Neptune from the roster of planets). However, as the only object previously described as a "planet" to be demoted to "dwarf planet", Pluto lost some status in the process.
Pluto itself doesn't care, and is unchanged by all this redefining of words. As is Pluto's satellite Charon. However, to be perfectly accurate, Pluto is Charon's satellite as well. This is because their relative masses are so similar that they orbit one another around a common center of gravity. So there they are, orbiting around one another in the outskirts of the Solar system, not caring about what anyone on distant Earth calls them. And Jon Coulton wrote a song about them.
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