Earth is the third planet from the Sun. It formed over 4.5 billion years ago and weighs approximately 13,170,000,000,000,000,000,000,000 pounds, or about 6.58 billion trillion tons. Earth is not a perfect sphere, but an oblate spheroid, “squashed” a little at the North and South poles. The equatorial diameter is about 7,926 miles, but the polar diameter is 7,899.86 miles. Earth’s atmosphere is categorized into five layers. From bottom to top, these layers are: troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Earth’s spinning molten core produces a magnetic “shield” (called the magnetosphere) around the planet, which largely protects the atmosphere from being blown away by solar winds.