Gradually, the matter in the flat disk began to form clumps under the influence of its own local gravitational forces. The largest of these masses swept up more and more debris as they orbited the early Sun, and they began to define a string of Planets. Terrestrial Planets, those nearest the Sun, were subjected to high temperatures and strong solar winds, so that most gases such as hydrogen, helium, and water vapors were swept out into space. Thus the inner four planets Mercury, Venus, Earth, Mars-- are dense, rocky places with a relatively how low content of gaseous elements.
The Earth's formation was probably typical of these planets. After the principle mass of the Earth had been collected together, additional rocks and boulders showered down in the great bombardment, adding matter and heat energy to the planet. Dense iron and nickel separated from lighter materials by the process of differentiation and sank to the center to form a metallic core. Most of the Earth's mass concentrated in the thickmantle, while the lightest elements formed a thin crust. The Moon, Earth's only large satellite, may have been formed when a planet-size body hit the Earth early in it history.
The solar system's outer Jovian planets --Juipter, Saturn, Uranus, and Neptune-- are quite different from the inner planets. Lying beyond the strong effects of solar heat and wind, they accumulated large amounts of gases such as hydrogen, helium, ammonia and water. These outer planets are thus giant balls of ices, with thick atmospheres and great frigid oceans of nitrogen, methane, and other compounds that are gases on Earth. Beyond the Jovian planets lies Pluto, a rocky body that is the smallest planet. All of the planets except Mercury and Venus, the closest to the Sun, have moons in orbit.
Interspersed with the planets and their moons are many other kinds of objects. Small rocky asteroids, most of which are concentrated in an asteroid belt between Mars and Jupiter, circle the Sun like miniature planets. Far outside the solar system, swarms of dirty snowballs called comets are concentrated in the Oort cloud. If a comet's distance orbit is disturbed, it may fall toward the Sun and create a spectacular display in the night sky. When a piece of interplanetary debris hits Earth's atmosphere, it creates a meteor, sometimes called a shooting star, which burns up with a fiery trail. Occasionally, a meteor fragment will hit the Earth and become a meteorite.
NOW IT IS TIME TO LEARN HOW THE EARTH EVOLVED!
