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Analyzing Theories: Formation of the Solar System Throughout the centuries, there have been many theories about the formation of the solar system. Below are the most commonly accepted theories. Nebular Hypothesis (1796, Immanuel Kant and Pierre Simon La Place) The protosun (original or first sun) contracted to form a huge rotating gas cloud (primeval solar nebula). As the gas cloud cooled, it contracted even more and the size of the Sun became smaller. The decrease in size caused an increase in the speed at which the Sun was turning and this increased the accelerating forces coming from the Sun. When the accelerating force from inside the Sun became stronger than the Sun’s forces of attracting, a ring separated from the main body of the Sun. As the Sun continued to contract and cool, more rings of matter, like ripples in a pond, moved out from the Sun. These gaseous rings gradually condensed into spheres that became planets. Tidal Theory (1785, George Louis de Button) A passing star raised a great tide on the Sun and released the gas that later formed the planets due to forces of acceleration. Planetesimal Theory (1900, Forrest Moulton and Thomas Chamberlain) The tidal action of a passing star released matter from the Sun. This matter cooled and by forces of acceleration large enough to overcome the gravitational pull of the Sun, formed planetesimals (small planets). These grew in size by picking up scattered material nearby and forming the planets. Combination/Accumulation Protoplanet Hypothesis (1950, G.P. Kuiper) The protosun formed at the center of the protosolar nebula. Rotation supported this nebula as it collapsed. Dust grains flew through the nebula and collected into a thin disk of material. Gravitational instability caused the disk to break up into small particles (planetesimals). Over time, the planetesimals formed the planets. Near the Sun, only the rocklike materials could survive the high temperatures. Further away from the Sun, icy and gaseous material could condense. For this reason, the inner planets are small, dense, and rocky, while the outer planets are large, less dense, and icy or gaseous. The thin disk of material from the collapsed nebula caused the orbits of all the planets to line in a flat plane around the equator of the Sun and all the planets except Venus and Uranus to rotate in the same direction. This theory also predicts that the moons around a planet should resemble a solar system in miniature, with all the moons lying within the equatorial plane of the planet, having circular orbits, and revolving and spinning in the same direction. Nebular Theory/Accretion Theory The Sun and the planets formed from a rotating disk of dust and gas. As the speed of rotation increased, the center of the disk flattened out and matter became more concentrated in the center of the disk, where the Sun eventually formed. The growth of the planets began as solid bits of matter began to collide and clump together through a process called accretion. The colliding matter formed small, irregularly shaped bodies called planetesimals. AS the collisions occurred, these planetesimals grew larger. As they grew in mass, they began to exert a gravitational pull on surrounding objects, thus adding still more mass and becoming planets. In the inner solar system, close to the Sun, temperatures were so high that only metals and silicate minerals could form solid grains. It was too hot for the ices of water, carbon dioxide, and methane to form. In the frigid outer reaches of the solar system, it was cold enough for these ices to form. Consequently, the inner planets are primarily rocky while the outer planets were able to increase in size by accumulating metals, silicates, and gases, eventually becoming large enough to even attract the lightest gases – helium and hydrogen.