It’s time to forget what they taught you in school about the solar system–it no longer has 9 planets. After years of controversy, Pluto has officially been stripped of it’s planetary status. It’s now officially a Dwarf Planet. The confusion arose because all these years the international astronomical community had no formal definition of what constitutes a planet! The solar system almost ended up with 12 planets, the new contenders being the asteroid Ceres, Pluto’s moon Charon, and a distant object called 2003 UB313.
So, what’s a planet and what’s a dwarf planet?
[Excerpt from The Knowledge News]
“Planets” Rule Their Space
According to the new definition, approved by the International Astronomical Union (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.
“Dwarfs” Don’t Rule Their Space
So what’s Pluto? The IAU has ruled that it’s a “dwarf planet.” According to the IAU, a dwarf planet in our solar system is a celestial body that meets four criteria:
- It orbits the sun.
- It’s round. (Same as above.)
- It’s not a satellite.
- It has not “cleared the neighborhood around its orbit.”
Under this definition, Ceres and 2003 UB313 are “dwarf planets,” too. Charon, which was slated for promotion from “Pluto’s largest moon” to “planet” will apparently have to rest content with its former title. (The rejected proposal would have revised the definition of “satellite” and made Pluto and Charon a “double planet,” since they orbit each other.)
I wonder how current literature and more important, school textbooks, will be updated.