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Unformatted text preview: Astronomy 101 - Test 2 Review SOLAR SYSTEM INTRODUCTION AND FORMATION The Solar System contains the Sun, the planets, their moons, rings, and various types of space debris such as asteroids and comets. The eight planets all orbit the Sun in the same direction, and almost all spin on their axes in this same direction. The shapes of the orbits are elliptical, but for all planets except Mercury they are almost circular. The Asteroid Belt exists between the orbits of Mars and Jupiter. The Solar System is also very flat. The planets divide well into two classes: the Terrestrials and the Jovians. The Terrestrials (Mercury, Venus, Earth, Mars) are closer to the Sun, are smaller, have a rocky/metallic composition, have higher average densities, spin slower on their axes, have few moons and no rings. The Jovians (Jupiter, Saturn, Uranus, Neptune) are the opposite in all of these regards. Their gaseous/liquid envelopes, which make up most of their mass, are composed primarily of hydrogen and helium, reflecting the composition of the cloud out of which the Solar System must have formed, and in fact the entire universe. To explain how our Solar System formed, we need a good theory since we did not see it happen. We also need to check our theory against other forming stars and solar systems, to the extent made possible by current observations. We must explain why the Solar System is so flat, why almost everything orbits and spins in the same direction, why there is only a handful of well-separated planets, and the Terrestrial-Jovian distinction. The best theory goes as follows: it began with a cold, relatively dense cloud (or more precisely, part of a much larger cloud) of interstellar gas which started collapsing under its own gravity. Such clouds are observed elsewhere, and various stages of the star formation process are seen in some of them. These clouds are observed to be rotating through use of the Doppler Shift. When a rotating object collapses, it rotates faster (like the ice skater). This is called conservation of angular momentum. The faster rotation makes the cloud collapse into a flattened disk, because working against the collapse is the tendency for gas to be flung outwards because of the faster rotation (like tossing pizza dough). All interstellar clouds consist of gas (about 98% of the mass), and dust grains (about 2%). This collapsed rotating disk of gas and dust is dense enough so that gas atoms frequently run into the dust grains and stick, causing them to grow. Eventually, the growing dust grains collide with others, often sticking together, leading to a larger piece. In this way, the earliest chunks of solid matter were built. This kept going until chunks were large enough (many km) so that their gravity became significant. The gravitational attraction of these so-called planetesimals enhanced their collision rates. Large planetesimals had stronger gravity, so collided more often, growing ever larger. The result was only a few large chunks left - the Terrestrial planets and the solid cores of the Jovian planets.large chunks left - the Terrestrial planets and the solid cores of the Jovian planets....
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