Every human being is unique: Each person has his or her own genetic makeup and personal history. On the other hand, people have many similarities: We all breathe air; we all consume food and water; and, under normal circumstances, we have common characteristics, such as two eyes, two hands, and ten toes, among other traits. To understand humans fully, biologists and physicians study our common features and then our individual peculiarities, just as psychiatrists and psychologists study our common behaviors as well as our behavioral differences.

The planets in our solar system also have similarities and differences that astronomers are learning to understand. For example, there are three basic groups of planets: terrestrial worlds (Mercury, Venus, Earth, and Mars), all roughly similar in size and chemistry to Earth; the much larger gas giant, or Jovian, worlds Jupiter and Saturn; and the planets intermediate in both size and chemistry, the ice giant worlds Uranus and Neptune.

In this first of two chapters on the rest of the planets, we explore the terrestrial worlds both individually and in comparison to each other and to Earth. The remaining four outer planets are presented in the next chapter.