Formation of Earth and Our Solar System - We can construct the formation history of our solar system by looking at regions where other stars are forming now. Star formation begins when a giant cloud of gas and dust collapses under its own gravity. As the cloud contracts, it begins to spin faster and settles into a diskshaped structure. We see these disc-shaped objects (called proplyds) in the Orion Nebula, where the new stars are forming today. Most of the dusty disk material drains toward the center where the density gradually increases until the enormous central pressure triggers nuclear fusion reactions and the star is born.
However, a relatively small fraction of the disk material is left behind in the form of icecoated dust grains. The icy mantles of the grains begin sticking together and eventually grow to meter-sized rocky boulders called planetesimals. The planetesimals collide and accrete into larger bodies that are tens of kilometers in diameter called protoplanets. Once the protoplanets clear a gap in the disk, they become bonafide planets and their orbits begin to stabilize.
The process of planet formation is messy. Not all of the planetesimals are accreted into planets. Millions of planetesimals remain as the leftover debris and are now the asteroids and ice-coated comets in our solar system. In the first hundred million years after the formation of the Sun, collisions between the leftover planetesimals and the planets were common. We see evidence for heavy bombardment by planetesimals on the surfaces of the moon and Mercury. The same types of collisions would have occurred on the surface of the Earth, however erosive processes have erased all except the most recent of these collisions.
About 100 million years after the formation of the Sun, the gravity of the planets and moons in our solar system had swept up most of the planetesimals. However, millions of these objects still remain in gravitationally stable orbits in the main asteroid belt of the solar system, in the Trojan asteroid belt, or out beyond Neptune and Pluto in the Kuiper belt. Illustrated in the sketch below is the location of the largest reservoir of asteroids in our solar system today. Earth is the only object in our solar system known to support life. Today there are over 1 million known species of plants and animals on Earth.
The materials that came together to form the Earth were made of several different chemical elements. Each element has a different density, defined as mass per volume. Density describes how heavy an object is compared to how much space the object takes up. After Earth’s early formation, the denser elements sank to the center. The lighter elements rose to the surface. You have probably seen something like this happen if you have ever mixed oil and water in a bottle. The water is denser than oil. If you put both in a bottle, shake it up, and then let it sit for a while, the water settles to the bottom and the oil rises up over the top of the water.
Today, the Earth consists of layers that represent different densities. Earth’s center is called its core. The core is made of very dense metal elements called iron and nickel. The outermost layer of the Earth is its crust. The crust is made mostly of light elements such as silicon, oxygen, and aluminum. More information on the different layers of the Earth is presented in the lesson on plate tectonics.
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