Chapter 8: Formation of the solar system

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Astronomy & Astrophysics
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Ian Shelton

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Chapter 8: Formation of the solar system 8.1 The Search for Origins What properties of our solar system must a formation theory explain? - In chapter 7 we discussed four features of our solar system and these features hold the key to testing any hypothesis that claims to explain the origin of the solar system - If the hypothesis fails to explain even one of the four features then it cannot be correct and if it successful explains all four we might reasonably assume it is on the right track - The four criteria of success of solar system formation theory” 1. it must explain the patterns of motion we discussed in chapter 7 2. It must explain why planets fall into two major categories: small, rocky, terrestrial planets, near the sun and large hydrogen-rich jovian planets farther out. 3. It must explain the existence of huge numbers of asteroids and comets and why these objects reside primarily in the regions we call the asteroid belt, the Kuiper belt, and the Oort cloud 4. It must explain the general patterns while at the same time making allowances for exceptions to the general rules, such as the odd axis tilt of Uranus and the existence of Earth’s large Moon - The theory can gain additional support ex. This theory can make predictions about other solar systems What theory best explains the features of our solar system? - 18 C scientists Immanuel Kant proposed that our solar system formed from the gravitational collapse of an interstellar cloud of gas - 40 years later Pierre Simon Laplace put forth the same idea independently because an interstellar cloud is usually called a nebula, their idea became known as the nebula hypothesis - The nebula hypothesis remained popular throughout the 19 C th th - By the early 20 C scientist had found a few aspects of our solar system that the nebular hypothesis did not seem to explain well - During much of the first half of the 20 C the nebular hypothesis faced stiff competition from a hypothesis proposing that the planets represent debris from a near collision between the sun and another star - According to this close encounter hypothesis the planets formed from blobs of gas that had been gravitational pulled out of the Sun during the near collision - Today the close encounter hypothesis has been discarded because it could not account for either the observed orbital motions of the planets or the neat division of the planets into two major categories (terrestrial and jovian) - Moreover, the close encounter collision between our sun and another star given the vast separation between star system in our region of the galaxy, the chance of such an encounter is so small that it would be difficult to imagine it happening even once in order to form our solar system and it could not account for the many other planetary system that we have discovered in recent years - The modification of the nebular theory has much evidence accumulated in favour of the nebular hypothesis that it achieved the status of a scientific theory ----the nebular theory - The nebular theory: scientist found that the nebular hypothesis offered natural explanations for all four general features of our solar system 8.2 The Birth of the Solar System Where did the solar system come from? - The nebular theory begins with the idea that our solar system was born from a cloud of gas called the solar nebular that collapsed under its own gravity - But where did this gas come from? It was the product of billions of years of galactic recycling that occurred before the sun and planets were born - Universe as a whole thought to have been born in the big bang which produced two chemical elements: hydrogen and helium. Heavier elements were produced later by massive stars and released into space when the stars died and then the heavier elements mixed with other interstellar gas that formed new generations of stars - Although recycling within the galaxy has probably gone on for the most of the 14 billion year history only a small fraction of the original hydrogen and helium has been converted into heavy elements - Spectroscopy shows that old stars have a smaller proportion of heavy elements than younger ones just as we should expect if they were born at a time before many heavy elements had been manufactured What caused the orderly patterns? - The solar nebular began as a large spherical cloud of very cold and very low density gas - This gas was probably o spread out perhaps over a region a few light years in diameter that gravity alone may not have been strong enough to pull it together and start its collapse - Instead, the collapse may have been triggered by a cataclysmic event such as the impact of a shock wave from the explosion of a nearby star (a supernova) - Once the collapse started the law of gravity ensured that it would continue….remember that the strength of gravity follows an inverse square law with distance because the mass of the cloud remained the same as it shrank the strength of gravity increased as the diameter of the cloud decreased since gravity pulls inward in all directions explains why the sun and the planets are spherical Heating, Spinning and Flattening - As the solar nebula shrank in size three important processes altered its density, temperature, and shape changing it from a large diffuse (spread-out) cloud to a much smaller spinning disk - Heating: the temperature of the solar nebular increased as it collapsed such heating represents energy conservation in action. As the cloud shrank its gravitational potential energy was converted to the kinetic energy of individual gas particles falling inward. o These 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 - Spinning: The solar nebula rotated faster and faster as it shrank in radius. This increase in rotation rate represents conservation of angular momentum in action. The clouds shrinkage made fast rotation inevitable o The rapid rotation helped ensure that not all material in the solar nebula collapsed into the center: the greater the angular momentum of a rotating cloud the more spread out it will be - Flattening: the solar nebula flattened into a disk. This flattening is a natural consequence of collisions between particles in a spinning cloud. The random motions of the original cloud therefore become more orderly as the cloud collapses changing the clouds original lumpy shape into a rotating flattened disk o Similarly collisions between clumps of material in highly elliptical orbits reduce their eccentricities making their orbits more circular - The formation of the spinning disk explains the orderly motions of our solar system today - The planets all orbit the sun in nearly the same plane because they formed in the flat disk - The direction in which the disk was spinning became the direction of the sun’s rotation and the orbits of the planets - Computer models show that planets would rotate in the same direction they formed which is why most planets rotate the same way today - The fact that collisions in the disk tended to make orbits more circular explains why mot planets in our solar system have nearly circular orbits - Collapsing clouds go through heating, spinning, and flattening - The heating that occurs in collapsing cloud of gas means that the gas should emit thermal radiation primarily in the infrared - Many of these young stars appear to be ejecting jets are thought to result from the flow material from the disk onto the forming star and they may influence the solar system formation processes - We expect flattening to occur anywhere that orbiting particles can collide which explains why we find so many cases of flat disks including the disks of spiral galaxies like the milky way, the disks of planetary rings and the accretion disks that surround neutron stars and black holes in close binary star systems 8.3 The formation of Planets - The planets started to form after the solar nebula had collapsed into a flattened disk of perhaps 200 AU in diameter Why are there two major types of planets? - Rocky terrestrial planets formed in the warm inner regions of the swirling disk while jovian planets formed in the colder outer regions Condensation: Sowing the Seeds of Planets - The general process in which solid (or liquid) particles form in a gas is called condensation –we say that the particles condense out of the gas. These particles start out microscopic in size, but they can grow larger with time - Different materials condense at difference temperatures o Hydrogen and helium (98% of the solar nebula). These gases never condense in interstellar space o Hyrodgen compounds (1.4% of the solar nebula). Materials such as water (H2O), methane (CH4) and ammonia (NH3) can solidify into ices at low temperatures (below about 150 K under the low pressure of the solar nebula) o Rock (0.4% of the solar nebula). Rocky material is gaseous at very high temperatures, but condenses into solid bits of mineral at temperatures between about 500 K and 1300 K, depending on the type of rock. (A mineral is a type of rock with a particular chemical composition and structure) o Metal (0.2% of the solar nebula). Metals such as iron, nickel, and aluminum are also gaseous at very high temperatures, but condense into solid form at higher temperatures than rock –typically in the range of 1000 K to 1600 K - Hydrogen compounds could condense into ices only beyond the frost line –the distance at which it was cold enough for ices to condense –which lay between the present-day orbits of Mars and Jupiter - ***Temperature differences in the solar nebula led to different kinds of condensed materials at different distances from the Sun, sowing the seeds for two kinds of planets o Within the frost line, rocks and metal condense, hydrogen compounds stay gaseous o Beyond the frost line, hydrogen compounds, rocks, and metals condense o Within the solar nebula 98% of the material is hydrogen and helium gas that doesn’t condense anywhere - 2 types of planets: planets born from seeds of metal and rock in the inner solar system and planets born from seeds of ice (as well as metal and rock) in the outer solar system How did the terrestrial planets form? - The process by which small “seeds” grew into planets is called accretion o early in the accretion process, there are many relatively large planetesimals on crisscrossing orbits. o As time passes, a few planetesimals grow larger by accreting smaller ones, while others shatter in collisions o Ultimately only the largest planetesimals avoid shattering and grow into full-fledged planets o As the particles grew in mass, gravity began to aid the process of their sticking together, accelerating their growth into boulders large enough to count as planetesimals, which means “pieces of planets” How did the jovian planets form? - The young jovian planets were surrounded by disks of gas, much like the disk of the entire solar nebula but smaller ins ize. According to the leading model, the planets grew as large, ice-rich planetesimals captured hydrogen and helium gas from the solar nebula. What ended the era of planet formation? - Vast majority of hydrogen and helium got swept into interstellar space by some combination of radiation from the young Sun and the solar wind –a stream of charged particles (such as protons and electrons) continually blown outward in all directions from the Sun. Although the solar wind is fairly weak today, observations of winds from other stars show that such winds t
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