The origin of the geosphere is the story of the birth of our planet. The story begins in an immense cloud of gas and interstellar debris called a nebula, like the one shown in Figure 2-2a. About 4.6 billion years ago, a relatively dense region in the cloud began to collapse under its own gravity, as depicted in Figure 2-2b. The denser and hotter center became the Sun. The dust, rocks, and gases swirling around the Sun coalesced to form the planets of our solar system. The rocky planetsMercury, Venus, Earth, and Mars-formed closer to the Sun,
and the large gaseous planets-Jupiter, Saturn, Uranus, and Neptune-formed farther away. The third planet from the Sun is Earth (Figure 2-3).