NASA
Nasa: Mission Science: What Is a Planet?
Find out about how planets have been classified historically, and what scientists use to study them.
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.
California Institute of Technology
The Discovery of Eris, the Largest Known Dwarf Planet
Until the discovery of Eris, Pluto was known as the largest Keiper belt object. This dwarf planet has replaced Pluto in size and is accompanied by a satellite that has been given the name Dysnomia. Learn about this distant world here...
Harvard University
Harvard University: The Solar System
These hands-on activities are a great way for students to gain perspective on the relative sizes and distances of each planet, the relationship between the sun and Earth, and much more.