Transcript: Earth, the third planet out from the Sun, is a special place. We live here, and Earth is still the only place known in the universe to harbor life. Related to that is the fact that Earth is the only planet in the solar system with an oxygen-rich atmosphere. It’s the largest and densest of the inner terrestrial planets, about nine thousand miles in diameter, a smooth sphere. If you could shrink the Earth down to the size of a billiard ball, it would feel as smooth as a billiard ball with a thin sheath-like atmosphere barely 50 kilometers thick. The age of the Earth is inferred to be about 4.5 billions years from radioactive decay of rocks.