Book excerpt: Geared toward students, this guide provides an overview of the small bodies that orbit the sun. This volume in the Greenwood Guides to the Universe series covers asteroids, comets, and dwarf planets--those small bodies that revolve the Sun--and provides readers with the most up-to-date understanding of the current state of scientific knowledge about them. Scientifically sound, but written with the student in mind, Asteroids, Comets, and Dwarf Planets is an excellent first step for researching the exciting scientific discoveries of the smallest celestial bodies in the solar system. The book will introduce students to all of the areas of research surrounding the subject, answering many intriguing questions. It defines a dwarf planet and explains why Pluto is one. It looks at how such small bodies form, what they are made of, and what kind of atmospheres might they have. And it asks--and answers--whether asteroids, comets, and dwarf planets present a hazard to the Earth or to spacecraft.