Why Pluto is not a Planet

This graphic, taken from "Windows to the Universe," depicts the size of all bodies to scale, but not their distance from each other.


Until 2006, Pluto was considered to be the smallest real planet. Now it is labeled as a mere "dwarf planet."

How did that happen?
Was Pluto ousted in a recall election by partisan scientists?
Did Pluto violate the Code of Conduct for All Decent Planets?

Well, members of the International Astronomical Union actually did get together to tighten up their classification system. During their August, 2006 meeting they set the definition of planets to be objects orbiting the sun which are "large enough so that their own gravity pulls them into the shapes of spheres" which also have enough gravitational force to clear smaller objects out of their orbit (source: Windows to the Universe Dwarf Planets page).

Pluto, although it orbits the sun and has enough gravity to be spherical in shape, is now considered to be a dwarf planet, because it doesn't have enough gravity to pull smaller bodies into itself or fling them out of its orbit.

Go to Windows to the Universe to learn more about our Solar System.