Chapter 8 - AST101.docx

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Astronomy & Astrophysics
Michael Reid

Chapter 8: Formation of the Solar System Our theory of solar system formation is important because it holds the key to understanding the nature of planets. 8.1 The Search for Origins  Development of any scientific theory is an interplay between observations and attempts to explain those observations What properties of our Solar System must a Formation Theory Explain? Hypothesis can only rise to the status of a scientific theory if it offers a detailed, physical model that explains a broad range of observed facts. If hypothesis fails to explain even one of the four following criteria then it can’t be right: 1. Must explain the patterns of motion discussed in Chapter 7 2. Must explain why planets fall into two categories: a. Small, rocky terrestrial planets near the Sun b. Large, hydrogen rich, jovian planets further out 3. 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, Kuiper belt, and the Oort Belt 4. Must explain the general patterns while at same time making allowances for exceptions to the general rules Our modern theory of the solar system formation has undergone little modification (can be applied to other solar systems) which gives us confidence that the basic formation theory is correct What Theory Best Explains the Features of Our Solar System?  1755 Immanuel Kant proposed that our solar system formed from the gravitational collapse of an interstellar cloud of gas , 40 years later so did Pierre-Simon Laplace (was unaware of Kant’s), known as Nebular Hypothesis (because interstellar cloud is usually called a nebula)  Close Encounter Hypothesis was the other theory around the same time (19 /20 th century) which proposed that planets formed from blobs of gas that had been gravitationally pulled out of the Sun during a near collision with another star o Doesn’t take into account: the observed orbital motions of the planets, division of planets into two major categories also stars colliding is very unlikely, which would mean planets are unlikely-instead planets are quite common  Nebular Hypothesis-Nebular Theory after modification (theory of our solar systems birth) 8.2 The Birth of the Solar System *likely to apply to the births of other solar systems as well Where did the Solar System Come From?  Solar Nebula: the cloud of gas that collapsed on itself due to its own gravity and was the beginning of our solar system Galactic Recycling Fig 8.1  Hydrogen and helium (trace of lithium) were the only elements present when the universe was young  Heavier elements were produced by stars (nuclear fusion) or the explosions that accompany stars death o When stars die they release much of their content back into space o This material can be recycled into a new generation of stars  The gas that made up the solar nebula was the result of billions of years of galactic recycling that occurred before our solar system was born  When our solar system formed only 2% of the original hydrogen and helium had been converted to heavier elements (other 98% stayed, our Sun still has this basic composition); The sun represents nearly all the mass in our solar system  This is why we are star stuff, because we and our planet are made of elements forged in stars that lived and died long ago Evidence from other Gas Clouds  Stars that appear to be in the process of formation are always found within interstellar clouds  Orion nebula: an interstellar cloud in which new star systems are forming  These stars have dense gas around them which can eventually lead to formation of planetary systems similar to ours What caused the orderly patterns of motion in our Solar System?  Solar nebula probably started as large, roughly spherical cloud of very cold and very low- density gas (over few light years in diameter), so large that gravity alone may not have been able to start the collapse o May have been impacted by shock wave of the nearby explosion of the star (supernova)  Once collapsed started, law of gravity ensure it would continue o The mass of the cloud stated the same as it shrank, the strength of gravity increased as the distance decreased (diameter decreased by half, force of gravity increased by factor of 4)  We find that the solar nebula should have become a spinning, flattened disk surrounding a central star (gravity is not the only physical law that affects the collapse of a cloud-that’s why it doesn’t stay in spherical shape) Heating, Spinning and Flattening As solar nebula shrank three important processes altered its density, temperature and shape, changing it from a large cloud to a much smaller spinning disk  Heating: temperature increased as it collapsed, as cloud shrank its gravitational potential energy was converted into kinetic energy, then was converted into thermal energy as molecules crashed into each other, sun formed in the centre, where temperatures and densities were the highest  Spinning: solar nebula rotated faster and faster as it shrank in radius, increase in rotation rate represents conservation of angular momentum in action, this rapid rotation insured that not all the material in the solar nebula collapsed in the centre, the greater the angular momentum of a rotating cloud, the more spread out it will be  Flattening: solar nebula flattened into a disk, natural consequence of collisions between particles in a spinning cloud, clouds may have clumps of gas within them, as the clumps collide, new clumps are formed with the average velocity of the clumps that formed- random motions of the original cloud become more orderly as it collapses which changes the original lumpy shape into a rotating, flattened disk 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 the planets would also rotate in this same direction as they formed, which is why most planets rotate the same way today Testing the Model Evidence does support our model of spinning, flattening and heating:  Computer simulations  Observing disks around other forming stars Fig 8.4 o The heating that occurs in a collapsing cloud of gas means that the gas should emit thermal radiation, we’ve detected infrared radiation from many nebulae where star systems appear to be forming today  By observing many other structures in the universe, we expect flattening to occur anywhere that orbiting particles collide 8.3 The Formation of Planets Why are there two Major Types of Planets? The terrestrial planets formed in the warm, inner regions of the swirling disk, while the jovian planets formed in the colder, outer regions Condensation: Sowing the Seeds of Planets  in the center of the of the collapsing nebula, gravity drew together enough material to form the Sun  in the surrounding disk, material was too spread apart for gravity alone to clump it together, so material had to begin clumping in another way and grow in size until gravity could start pulling it together into planets  planet formation required the presence of “seeds”-solid bits of matter from which gravity could build planets  Condensation: the general process in which solid (or liquid) particles form a gas  On the basis of their condensation properties, the ingredients of the solar nebula fell into four major categories Table 8.1: o Hydrogen and Helium gas(98% of solar nebula), never condense in pressures of the nebula o Hydrogen compound (water, methane, ammonia) (1.4% of the solar nebula), can solidify into ices at low temperatures (<150K) o Rock(0.4% of the solar nebula)gaseous at very high temperatures, condenses into solid bits between 500K and 1,300K o Metal(0.2%) metals such as iron, nickel and aluminum also are gaseous at very high temperatures but condense into solid form at higher temperatures than rock Since the nebula was mostly made up of hydrogen and helium most of its mass did not condense. Close to where the Sun was forming, where the temperature was above 1600K it was too hot for any material to condense. Near what is now Mercury’s orbit, the temperature was low enough for metals and some types of rock to condense into tiny particles, but other types of rock and all the hydrogen compounds remained gaseous. More types of rock and metal could form at the distances from the Sun where Venus, Earth and mars would form. Region where the asteroid belt would form, temperatures were low enough for to allow carbon-rich minerals to condense. Hydrogen compounds could only 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)  Frost line marked the key transition between the warm inner regions of the solar system where terrestrial planets form and the cool out regions where jovian planets formed  Beyond the frost line, where it was cold enough for hydrogen compounds to condense into ices, the solid seeds were built of ice along with metal and rock Planets born from seeds of metal and rock in the inner solar system and planets born from seeds of ice in the outer solar system. How did the terrestrial planets form?  Because rock and metal made up such a small amount of the solar nebula, the terrestrial planets achieved only relatively modest size  Accretion: the process by which small “seeds” grew into planets o began with the microscopic solid particles that condensed from the gas of the solar nebula, they orbited the forming sun at nearly the same speed as neighbouring part
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