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Chapter 6

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University of Toronto Mississauga
John Lester

Astronomy Ch. 6 - Survey of our Solar System (Oct. 24) Sun - Sun is the heart of the solar system - Sun’s radiation warms all the objects in the solar system - Sun’s gravity holds the solar system together - But, we will ignore the Sun (it is covered in AST201 as the nearest star) to focus on everything else in the solar system How large is the Solar System? - Recall from Ch 1, “edge” of the solar system is about half way to the nearest star - measured using parallax - 100,000AU = 1.5 X 10^13 km - “edge” varies because the Sun and the nearby stars move around - This is far beyond Pluto or the new dwarf planets (“only” about 100 AU) or the comets Major Planets - Planets like Earth - “terrestrial” planets = Mercury, Venus, Earth and Mars - close to the Sun → warm - small diameters → small volumes - small masses (Earth is most massive) - large average densities = mass/volume ≈ 5 g/cm^3 - note: rock ≈ 3 g/cm^3 → rock + metals - few moons - Planets like Jupiter - “jovian” planets = Jupiter, Saturn, Uranus and Neptune - far from the Sun → cold - large diameters → large volumes - large masses: 14.5 to 318 X Earth’s mass - small average densities ≈ 1 g/cm^3 - note: water ≈ 1 g/cm^3 - many moons Dwarf Planets (New Class) - Pluto (1930), Quaoar (2002), Sedna (2004), Orcus (2004), Eris (2005), Makemake (2005) + ... - beyond Neptune → very cold - tiny diameters (less than Earth’s Moon) - tiny masses (less than Earth’s Moon) - medium average densities ≈ 2 g/cm^3 ▯ ▯ → contain a mixture of ice/rock Other Members - Small rocky objects - Asteroids (minor planets) Nov. 18 Clues to the Formation - We are trying to understand something that took place 4.5 billion years ago - What clues can we see today that give us information about how the solar system formed long ago? - The general properties of the solar system, things that were caused by how the solar system started Systematic Properties of Orbits - All large objects orbit counterclockwise when viewed from above Earth’s (and Sun’s) north pole - All large objects have nearly circular orbits - just slightly elliptical - Most larger objects spin in their orbit direction = counterclockwise - Most orbits are nearly align with the Sun’s equator = a solar system’s plane - Most of the larger moons share these traits: - counterclockwise orbits - counterclockwise spin - aligned above their planet’s equator Systematic Physical Trends - Rocky terrestrial planets are all close to the Sun - Gas jovian planets are all far from the Sun - Icy dwarf planets are all very far from the Sun - Properties of the asteroids and comets Deviations from the Trends - Most planets spin counterclockwise, same as their orbit direction, but not Venus and Uranus - Comets have very elliptical orbits - Most moons are much smaller than their planet, but Earth’s Moon is similar in size → Theories of the Solar System must be flexible enough to handle these deviations Theories of Formation Nebular Theory - Kant ≈ 1755, Laplace ≈ 1800 - Nebula (cloud in Latin) of gas (H, He, ...) with a small amount of dust - Nebula’s diameter ≈ 200,000 AU - Pulled together by its own gravity to form the Sun - planets, asteroids and comets form around it in a flattened disk Collision Theory - ≈ 1900 - Our sun formed without planets - Later Sun “collided” with another star = come close enough to have tidal stretching from each other’s gravity - Tides pull matter from each star - Stripped-off matter condenses under its own gravity to form planets around our Sun and around the other star Nov. 21 Problems with Collision Theory 1. Chances of a collision are extremely small - the Sun’s volume ≈ D^3 = 10^18 km^3 - the volume of the solar system (region with no other star) ≈ (200,000 AU)^3 ≈ 10^40 km^3 - chances of collision = 1/10^22 - maybe one time in the whole Universe 2. Sun should be much older than Earth - not what we find 3. Hot gas stripped from a star will evaporate, not condense → Reject the collision theory Support for Nebular Theory - In 1755-1800 no gas clouds were known - With photography (since 180) clouds of hot gas were detected - With radio telescopes (since 1950) and infrared telescopes (since 1960) clouds of cooler gas have been discovered - Detect stars forming within these gas clouds Solar Nebula → Solar System - The nebula began to contract fo
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