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Astro 1:HW 6 Solutions (Winter 08)

Astro 1:HW 6 Solutions (Winter 08) - UCSB Winter 2008 Astro...

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UCSB Winter 2008 Astro 1 - Homework #6 Solutions 2/29/08 1 Consider the formation of the solar system. What are the two reasons that the outer protoplanetary cores (those of Jupiter, Saturn, Uranus, and Neptune) were able to attract and gravitationally bind vast quantities of Hydrogen and Helium, giving these planets very large masses and low densities relative to the terrestrial planets Mercury, Venus, Earth, and Mars? Hint: this involves the concepts of the superabundant light gases H and He, the ices such as water and carbon dioxide, and the rocky and metallic elements like Iron, Manganese, etc. In the Nebular model of the formation of the solar system, stars and planets are formed out of a nebula of gas and dust that contracts under gravitational attraction. (If it is to form a solar system, the nebula must have some net angular momentum. This allows for material to orbit the protosun and eventually form planets instead of just falling into it.) There are two major reasons that small terrestrial planets form close to the sun and large gaseous planets, known as the Jovans form farther away. First remember as in the hint that most of the matter in the universe, including in the nebula in question, is made up of the two lightest elements, Hydrogen and Helium. These were formed just after the so-called big bang and Helium is the primary byproduct of all stars. The heavier elements can only be formed toward the end of a star’s life, and less of the heavier the elements are created than lighter ones. This means that there is much more material available to form gaseous planets than solid ones. The reason that the terrestrial planets (consisting almost entirely of a solid core) are close to the sun and the Jovans (composed mostly of helium and hydrogen) are far from it, is that the protosun is hot, even hotter than the sun is today. The heat near the protosun vaporizes the lighter elements while the heavier elements stay in their solid state (rocks). The solid elements can stick together and form rocky planets while the gas can’t stick together. Far away from the protosun, where things are much cooler the lighter elements can also be in a solid state (ice) and stick together.
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