Wait just one minute here...
Charon, moon of Pluto, gets to be a planet because it's round and over 800 km wide, but Ganymede - moon of Jupiter - is still a moon, despite being more than 4 times the size?
...there seems to be an exception for Charon because it's less of a "moon" and more of a junior partner in the Pluto-Charon "double planet". For all the talk of keeping Pluto a planet for the schoolchildren, this is a pretty arcane reason to deny the Galilean moons of Jupiter their rightful title.
I think Ganymede is being robbed. I also think this is an important enough issue to post twice about. Finally, I think I have way too much spare time today.
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Good observation - apparently the scientists have determined that Pluto and Charon are a binary planetary system - very cool stuff is happening at the conference
The reason the huge moons aren't now classified as planets is because they properly orbit their planet.
OTOH, Charon doesn't actually orbit Pluto.. because it is so close in size to Pluto, the center of gravity between the two is actually in space.
Thus, they are two planets orbiting each other and orbiting the sun... "Double Planet"
For the same reason our own moon, which is bigger than Pluto, is still a moon (satellite) because the center of gravity is within the Earth.
It's actually a fairly nice elegant and common sense solution.
This new planetary definition follows in the legacy of Esperanto. It's built around logical and unambigious rules... but in the end, the public will ignore it and use what they use already.
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