The day we lost Pluto
or (And your little dog, Goofy, too!) It’s official, the International Astronomical Union (IAU), founded in 1919, has defined (I never had that much time in my life to define something -- most of my identify and define tests were given in the time frame of 40 minutes, slackers.) what a planet is: A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. According to this definition, Pluto has been demoted from a planet to a dwarf planet. A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. Pluto is a dwarf planet by the above definition and is ...
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