University of California
Ucla: What Makes a Planet
This detailed essay defines planets and compares the characteristics to those of Pluto. Orbital graphs and background information clarify the change in Pluto's categorization. A copy of IAU's Resolution 5 is also contained in this essay.
ClassFlow
Class Flow: Pluto Redefined
[Free Registration/Login Required] This flipchart briefly describes the new definition of planet as determined by the International Astronomical Union.