Explains how Pluto was discovered, and provides information about the debate over whether or not Pluto is really a planet. Includes color photographs, fact boxes, a glossary, and additional resources.

"Investigate!"

The old solar system -- A hidden planet -- Too far to see -- A lucky accident -- Not like other planets -- How big is a planet? -- An official definition -- So what is Pluto? -- Other dwarf planets -- Love for Pluto -- A planet again?.