The Earth is the only planet known to support life. Although larger than the other inner planets, Earth is much smaller than the gas giants found in the outer solar system. Earth is a terrestrial planet with liquid water covering roughly 70 percent of its solid surface.
The third planet from the Sun, the Earth has an atmosphere that contains both free oxygen and liquid water, two key elements needed to support life. The Earth rotates on an axis, an imaginary line running through the planet. The axis of the Earth is tilted to a degree that varies the amount of light and heat energy each hemisphere receives. These seasonal changes take place due to the shift in position experienced by the southern and northern hemispheres at different points in the Earth's orbit.
Earth's surface is known as the crust. The crust is composed mostly of alumino-silicates and makes up just 1 percent of the planet's total volume. While most of the planet's surface is covered in water, the crust also has several tectonic plates that float above the molten asthenosphere. These plates are able to float on the planet's crust because they have a lower density than material found within the planet's mantel.