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Chapter 4 -- The Solar System

Chapter 4 -- The Solar System - GEO 1013 THE THIRD PLANET E...

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GEO 1013 – THE THIRD PLANET E. R. SWANSON – SPRING 2009 ORIGIN OF THE SOLAR SYSTEM Introduction How about this scientific question? What is the origin of our star and the motley collection of planets (including Earth), all the moons, asteroids, meteors and comets held in orbit by the Sun? Well, let’s take a look at the evidence. Most of these objects have the same shared orbital and rotational motion (counterclockwise). It really looks, therefore, as if the whole solar system formed together in some manner. The hypothesis should, therefore, account for the fact that all of the planets revolve around the sun in the same counterclockwise direction and that the planetary orbits lie in nearly the same plane. This counterclockwise motion also extends to how the planets spin, as all but Venus and Uranus rotate in the same direction that they revolve around the Sun. Most moons also revolve in a counterclockwise direction. There seems to be one right way to move in this solar system. The mechanism that formed the solar system must also explain why the solar system is divided into an inner group of terrestrial planets , small earth-like planets rich in iron, silicon and other relatively heavy elements; and an outer group of giant, low density Jovian (Jupiter like) planets that are probably like the Sun in composition but without the nuclear fusion and heat. Finally, most of the mass of the known solar system, 99.9%, is found in its central region, the Sun, and yet the Sun contains only about 1% of the solar system’s angular momentum. In other words, if 99.9% of the mass of the solar system had move inward to form the Sun, (and angular momentum was conserved), the Sun should be spinning much faster than it does. The Nebular Hypothesis As indicated above, all this solar system order suggests that everything formed together. The age of the oldest moon rocks and meteorites suggest that it started sometime prior to 4.6 billion years ago. Scientists hypothesize that about 5 billion years ago the solar system was forming from an enormous, slowly rotating hydrogen-rich cloud, a cloud that also contained all of the heavier elements that we have today. These elements were created in earlier stars, some ending with supernova explosions, and all of this a part of an evolving universe that began billions of years earlier. Slowly, interstellar dust seems to have been swept into a huge, slowly rotating cloud or nebula. As it continued to contract under its own gravity, the nebula became heated and its rotational motion increased so that it flattened to resemble a spinning disk (or Frisbee) of material. Gravity pulled most of the material (99.9%) to the center where it became our sun, but a number of eddy-like concentrations of material or rings of material also accumulate and begin the planetary formation process.
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