Of the eight planets in the solar system, the fifth-largest planet is Earth. This planet formed nearly 4.5 billion years ago. At a distance of 93 million miles, Earth is the third planet from the sun. Along with Mars, Venus and Mercury, Earth is an inner and terrestrial planet.
Terrestrial or rocky planets have solid surfaces. Earth's core consists of mostly nickel and iron. Unlike the other three terrestrial planets, Earth is inhabited and has the right conditions to support life. However, scientists believe that life may have existed on Mars in the distant past.
The Earth's period of rotation is 24 hours, and its period of revolution around the sun is 365 days. Earth's atmosphere consists of 78 percent nitrogen, 21 percent oxygen and small amounts of water and gases.