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Astronomy & Astrophysics
Michael Reid

Chapter 8: Formation of the Solar System  Clues of formation point to a common origin for the Sun, planets, moons, asteroids and comets that orbit it. 8.1 The Search for Origins  A hypothesis for the formation of the solar system must explain: 1. Patters of motion within solar system 2. Why we have 2 types of planets (Terrestrial and Jovian) 3. Why there are a large number of comets and asteroids 4. General patters as well as exceptions in solar system  If the hypothesis fails to explain one of the 4 features, then it cannot be correct. If it explains all 4, then we can assume that we are on the right track.  A theory can gain support if it makes predications that are borne out by new observations or experiments. th  18 century scientists proposed hypothesis that blossomed into our theory for the origin of the solar system. Immanuel Kant and Pierre-Simon Laplace proposed out solar system formed from the gravitational collapse of an interstellar cloud of gas (or nebula) = the nebular hypothesis  The close encounter hypothesis (= the planets formed from blobs of gas that had gravitationally pulled out of the sun during a near-collision with another star) competed with the nebular hypothesis 8.2 The Birth of the Solar System  Our solar system was born from a cloud of gas called a solar nebula which collapsed under its own gravity  This cloud was the product of billions of years of galactic recycling that occurred before the Sun and planets were born  When solar system formed, only a fraction of the original hydrogen and helium had been converted to heavier elements (gas forming nebula made up of 98% hydrogen and helium and 2% all other elements)  Spectroscopy shows that old stars have a smaller proportion of heavy elements than younger ones  Solar nebula began as a large, roughly spherical cloud of very cold and very low density gas  Gas was initially very spread out (over a region of a few light-years in diameter)  Gravity probably not strong enough to pull gas together -> collapse probably initiated by a cataclysmic event like the impact of a shock wave from the explosion of a nearby star (= supernova)  When collapse started, gravity made sure it continued  Strength of gravity increased as diameter of cloud decreased  Idea that gravity pulls in all directions explains why Sun and planets are spherical  As solar nebula shrank, 3 processes altered its density: 1. Heating: The temperature of the nebula increased as it collapsed. The particles crashed into one another, converting the kinetic energy of their inward fall to the random motions of thermal energy. The Sun formed in the center, where the temperatures and densities were highest 2. Spinning: The solar nebula rotated faster and faster as it shrank in radius (= conservation of angular momentum). The rapid rotation helped ensure that not all the material in the solar nebula collapsed in the center as the greater the angular momentum of a rotating cloud, the more spread out it will be. 3. Flattening: The nebula flattened into a disk, a natural consequence of collisions between particles spinning in a cloud. The random motions of the original cloud therefore become more orderly as the cloud collapsed, changing the cloud’s original lumpy shape into a rotating, flattened disk.  Planets all orbit Sun in same plane because they formed in the flat disk.  Heating that occurs in a collapsing cloud of gas means that the gas should emit thermal radiation (especially infrared). Today, we’ve detected infrared radiation coming from nebulae where star systems appear to be forming.  We expect flattening to occur anywhere that orbiting particles can collide which explains why we find so many cases of flat disks in the universe (galaxies, solar systems…) 8.3 The Formation of Planets  Terrestrial planets formed in warm, inner regions of the swirling disk while Jovian planets formed in the cooler, outer regions  In the center of the collapsing solar nebula, gravity drew together enough material to form the Sun  In the surrounding disk, the gaseous material was too spread out for gravity alone to clump it together so planet formation required the presence of “seeds” or solid bits of matter from which gravity could ultimately build planets  Formation of “seeds”: When the temperature is low enough, some atoms or molecules in a gas may bond and solidify (= condensation is the process in which solid particles form in a gas -> the particles condense out of the gas)  Different materials condense at different temperatures: - Hydrogen and helium (98% of nebula) never condense in interstellar space - Hydrogen compounds (1.4% of nebula) can solidify into ices at low temps - Rock (0.4% of nebula) condenses into solid bits of mineral at temps between 500 K and 1300 K - Metal (0.2% of nebula) condenses at higher temps than rock, between 1000 K and 1600 K  Most of nebula never condensed and remained gaseous. However, other materials could. Around Sun, temp was to high for materials to condense but near what Mercury’s orbit is now, temps were lower than 1600 K and metals as well as some types of tock could condense into tiny, solid particles. Further we went from Sun, the more materials could condense.  In region where asteroid belt now is, temperatures were low enough to allow dark, carbon-rich minerals to condense, along with minerals containing small amounts of water. Hydrogen compounds could condense into ices only beyond frost line (which lay between present-day orbits of Mars and Jupiter)  Frost line marked transition between warm inner regions of solar system (were Terrestrial planets formed) and cool outer regions of solar system (where Jovian planets formed).  Inside frost line, only metal and rock condensed into solid “seeds” so Terrestrial planets were made of rock and metal  Outside frost line, hydrogen compounds could condense into ices so Jovian planets were made of ices, metal and rock  Hydrogen compounds were almost 3 times as abundant in nebula as metal and tock combined so the total amount of solid material was far greater beyond frost line than within it (this is why Terrestrial planets are smaller than Jovian ones)  Process by which small seeds grew into planets is called accretion -> microscopic solid particles condensed from gas of solar nebula and began orbiting the Sun. The par
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