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AS101 Week 10 Lecture 1.docx

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Patrick Mc Graw

AS101 Week 10 Lecture 1 Review: Solar Nebula Model  Solar system formed from a collapsing cloud.  Spin of the solar system (and all the objects in it) comes from the spin of the original cloud.  Spin is also responsible for the flatness of most of the solar system in the plane of the ecliptic. (Note: this is the same process that gives the Milky Way galaxy its mostly flat shape.)  Chemical composition and size of planets changes with distance from the sun.  This has to do with changes in temperature with distance from the sun: farther away = colder, and therefore more solids available for forming planets.  One important boundary was the ice line: the distance at which temperatures were low enough to solidify water.  Process of planet formation would have stopped soon after the sun “turned on” and drive away most of the remaining gas + dust.  So, if this is typical for our solar system, it should be typical for other solar systems too Review: Two main ways of detecting extrasolar planets I. Doppler method  Most successful methods are Doppler method and transit method  Doppler method: Detect the slight wobble of the star as the planet's gravity pulls on it.  To detect the wobble, look for regularly recurring changes in the star's radial velocity (motion toward or away from us) using the Doppler effect.  This allows us to figure out the orbital period (from the period of the wobble) and the mass of the planet (from the size of the wobble.) Review: Two main ways of detecting extrasolar planets I. Transit method  Transit method: Detect recurring changes in the star's brightness as the planet moves in front of it.  This allows us to find the planet's orbital period and its size (from the amount of light it blocks.)  Transit only works if the planet's orbit happens to be almost edge-on to our line of sight.  By measuring the amount of light blocked by the planet we can measure the diameter of the planet Review: Extrasolar Planets  Detection of planets is easiest when the planets are massive and close to the star (so that their gravitational pull is strong and their orbital period is short).  Therefore the planets we have found so far are probably a very biased sample: lots of “hot Jupiters.”  If our understanding of how solar systems form is correct, these should be rare because large planets can normally only form farther away. What do we know about our planetary neighbours and how do we know it?  Our own planet is a good start  How do we know about the properties of Earth (composition, mass, etc)? Learning about Earth  Eratosthenes figured out the size a couple of millennia ago. o Eratosthenes found the diameter of the Earth by looking at the angle of the sun on the same day from 2 different places on earth and using geometry to figure out the circumference and diameter of the Earth  We can also find the mass. How?  We know the orbital period and average distance of the Moon from the Earth...  So Newton's version of Kepler's third law allows us to calculate the mass.  Then we can divide mass by volume to get density. Newton's Generalization of Kepler's 3rd Law p^2 = ((4pi^2)/GM) * a^3 Where M is the mass of the sun (or other central object)  This can be used to figure out the mass of the Sun.  Similarly, the orbital periods and distance of Earth's or other planets' moons can be used to figure out the planet's mass.  Fine print: Actually, the M in this equation should be the combined mass of the two objects, but the planets' masses are much smaller than the sun's. What is the Earth made of?  We can get samples of rocks from near the surface...  We can also divide the mass by the volume to get the density, which gives us at least some clues to what the planet as a whole is made of. (This is true for any planet or other object.)  How else can we learn what's inside? Some ways of learning about Earth's interior  Seismology: When an earthquake happens, vibrations travel through the Earth in the form of s waves and p waves  P waves = pressure waves, longitudinal waves, like sound waves (compressional)  S waves = Shear waves, don’t travel well through liquid/gas  Both types of waves get refracted as they travel through different materials.  s waves cannot travel through liquids at all--- so the fact that s waves do not travel through the middle of the Earth provides evidence that part of the core is liquid.  Earth's magnetic field also provides indirect evidence that part of the core is liquid metal:  combination of convection and the Earth's rotation creates the magnetic field.  Features of the surface due to plate tec
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