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Science of Our Planet Science of Our Planet What is planet Earth? Planet earth is one of nine planets in the Milky Way galaxy that orbit the Sun. The Earth fully rotates on its axis once a day and orbits the Sun once every 365.24 days. It is estimated that planet Earth is 4.5 billion years old (“Time’s Origins”, 2003). The Earth is composed of layers of rock which is usually a mixture if various minerals. Each mineral has its own chemical compound that allows it to react differently to gravity, motion, and heat. The outer layer of Earth, known as the crust, is made up of low –density silicates. Silicates are minerals that contain mostly silicon and oxygen. Also, the crust varies incredibly in thickness. The crust has shown to be thinner under oceans and thicker under continents. The next layer, known as the mantle, is composed of dense silicates. Although the mantle is denser than the crust, it is not as dense as the core.
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