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. The inner layer or core is composed of dense materials such as nickel and iron. The reason for layers is due to a process known as differentiation. Differentiation is the process in which gravity separates materials by density (Bennet, J., Donahue, M., Schneider, N., Voit, M., 2002, p. 229).

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The outer most layer of Earth is often referred to as the biosphere. The biosphere is the layer of living organisms that cover the surface of the planet and it readily distinguishes the planet from all others in the solar system ("Biosphere", 2003). Due to its flowing water and oxygen, Earth is the only planet in the galaxy that is naturally habitable by humans. It is estimated that life has existed on Earth for 3.5 to 4 billion years.
The Earth’s surface developed much like the other planets in the galaxy, through four major geological processes. The first process is known as Impact Cratering. Impact craters are the result of comets or asteroids impacting with the surface of a planet. Second, is the process known as volcanism. Volcanism is the result of molten rock and lava spewing from the Earth’s interior to its surface. Third, is the process of tectonics. Tectonics is the disruption of a planet’s surface by internal stresses. Fourth and finally is the process of erosion. Erosion is the result of wearing down or building up geological features by wind, water, and other planetary weather.
Between the crust and mantle of the Earth, a layer of rock known as the lithosphere exists. The lithosphere is a hard layer of rock that is composed of mostly crust but does include a small portion of the mantle. The lithosphere is broken into plates and allows the softer rock, known as the asthenosphere, to move and flow. This process results in the lithosphere gradually moving and "creating the phenomenon known as continental drift" (Bennet, J., et al., p. 230).
The study of the Earth’s interior is aided by measuring the seismic waves that are generated from earthquakes. The two types of seismic waves, primary and secondary, are measured using a device known as a seismograph. Seismographs are located throughout the planet and allow the comparison of measurements and recordings to determine the path of the seismic waves. Primary or P waves are the faster of the two and are similar to sound waves. Additionally, P waves have the ability to travel through liquid, gas, and rock. Secondary or S waves, have the ability to travel through rock but are unable to travel through gas or liquid. Using this information, it has been determined that some part of Earth’s interior is liquid. While P waves reach the side of the world opposite of an earthquake, S waves do not.
In closing, the question "What is Planet Earth?" is not so simple to answer. Earth is a mixture of minerals, elements, and life that continues to evolve. The question should not be what is planet Earth, but what will planet Earth become?

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