inner planets

Rocky planets closest to the Sun; located inside the asteroid belt.

asteroid belt

Circular region between Mars and Jupiter containing the greatest number of asteroids; marks the boundary between the inner and outer planets.

Mercury

The planet closest to the Sun; devoid of atmosphere, heavily cratered and marked by extreme variations in temperature (-300°F to 800°F).

Venus

Second planet from the Sun; its density and chemical composition are similar to those of Earth.

Mars

Fourth planet from the Sun; its crust contains iron oxide, giving Mars its reddish color.

Earth

Third planet from the Sun, inhabited by humankind; up to now, the only planet with evidence of life.

1 astronomical unit

Unit of distance equal to the mean distance between Earth and the Sun, equivalent to about 93 million mi.