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.
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