there was 9, then we thought maybe 12?
But now the # is 8.
Planets" Rule Their Space
According to the new definition, approved by the IAU, a planet in our solar system is a celestial body that meets three criteria:
* It orbits the sun.
* It's round. More technically, it "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape."
* It "has cleared the neighborhood around its orbit." Basically, it dominates its part of space.
That last criterion revokes Pluto's planetary license--little Pluto doesn't dominate anything. The only planets are Mercury (the smallest), Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
