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UniverseSeventh Edition
Chapter 8:Comparative Planetology II:
The Origin of Our Solar System
Copyright © 2005 by W. H. Freeman & Company
Roger A. Freedman and William J. Kaufmann III
Solar System attributes
• Rocky, small terrestrial planets, Gaseous (hydrogen and helium), giant jovian planets
• Planets orbit sun in the same direction
• Terrestrial planets orbit closer than jovian planets
Abundances & heavy elements• Hydrogen and helium 98%, heavy elements
2%
• Why? Big bang formed lighter elements and stars produced heavier elements.
• Smaller abundances means smaller planets (Item 1)
Radioactive dating and the age of Solar System
• Radioactivity
• Rocks on Earth ~ 4 billion years
• Rocks on moon ~ 4 billion years
• Meteorites ~ 4 billion years
• Hence, Solar system age ~ 4 billion years !!!
Proto planetary disk
Gravitational energy of contracting gas to thermal energy is “Kelvin-Helmholtz contraction”
• Item 2: Planets orbit sun in the same direction
Planets formed by accretion of planetesimals and gases
• For a given pressure, “condensation temperature” determines gas or solid phase.
• Water, methane, ammonia ~ 100 K
• Rocky substances ~ 1500 K
• Hydrogen, helium ~ 0K
• So hydrogen and helium always are in gas phase.
• Planetesimals: Chunks of rocks coalesced to form asteroidlike objects (~ 1 km)
• Protoplanets: Planetesimals collided to form moon size objects.
Chemical Differentiation
Core accretion of outer planets
• The gravity of Jovian planets sent most of the asteroids either away from SS or crashed into planets to form craters.
• Kuiper belt objects ( ex : pluto) formed beyond jupiter but sent away farther by gravity.
• Some went even as far as 50,000 AU and formed “Oort cloud”.
• Comets come from Kuiper belt or Oort cloud
T Tauri wind
Extrasolar planets