The third planet from the sun is, in scientific terms, quite similar to the first two. In fact, the four planets of the inner solar system (Mercury , Venus, Earth and Mars) all share rock and metal as their primary ingredients. Each of these so-called terrestrial planets has a solid surface, unlike the gaseous planets of the outer solar system. Perhaps Earth's most distinguishing factor, at least from our point of view, is the presence of water, which contributed to the formation of life some 3,000 million years ago. Most of us ought also to be fond of Earth's unique atmosphere, rich in life-sustaining nitrogen and oxygen. The Earth's surface is rotates about its axis at 1,532 feet per second - slightly over 1,000 miles per hour - at the equator, and the planet zips around the sun at more than 18 miles per second.