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.

This site contains copious information about Pluto. The site contains descriptive and statistical information, as well as a detailed article about the search for trans-Plutonian planets and the "Nemesis" hypothesis.

Find out about how planets have been classified historically, and what scientists use to study them.

A detailed overview of the history, mythology, and current scientific knowledge of each of the planets and moons in the solar system.

Students explore the outermost planets of our solar system: Saturn, Uranus and Neptune. They also learn about characteristics of Pluto and its interactions with Neptune. Students learn a little about the history of space travel as well...

Amazing Science looks at the Solar System with this FAQ site and lesson plan.