Posted by BC on August 21st, 2008
When is a “planet” not a planet, but an object in some other category? It depends on with which group you are associated.
The IAU defined a “Planet” as having to : 1) Orbit a sun; 2) Have enough mass to draw itself into a round shape; and 3) Have enough mass to clear out its orbit through impact or scattering.
The definition has its flaws. What about Trojan asteroids that accompany the giant planets? Jupiter and Saturn still have work to do. And “round”? Does “round” mean “with no sharp edges”?
So the terms “dwarf planet” and “plutoid” come into play, but that’s another story. You can learn more about the planet debate and see more arguments, for and against, on the definition of “planet.”