What happened in the last week? First the big news that stated the AU folks put the solar system at 12 planets, adding Charion, Ceres and Xena. Now a week later we are down to 8 planets, making those 3 and Pluto the new term "Plutons". What happened? Is this the same organization who just made a U-turn on their decision from last week or did a different group decide this?
Cowboy: Why you doin' this, Doc?
Doc Holliday: Because Wyatt Earp is my friend.
Cowboy: Friend? Hell, I got lots of friends.
Doc Holliday: ... I don't.
They added an extra criterium to the definition when it came to voting on it. The definition they agreed on is exactly the same as the one proposed last week except for that one detail, and it just happens to prevent Pluto et al being called planets.
I find it interesting that this topic has taken up so much time among astronomers. I suspect a conspiracy among text-book makers and people who develop museum exhibits must be behind the movement! Think of all the things that now will have to be revised!
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planet
The recently adopted IAU resolution states that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:
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 neighbourhood around its orbit.
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) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
So by this official definition there are exactly eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto, Ceres and 2003UB313 are "dwarf planets" with a potentiall large number of additional object falling into this category in the near future.