Roger A. Freedman and William J. Kaufmann III
Universe
Seventh Edition
Chapter 8:
Comparative Planetology II:
The Origin of Our Solar System
Copyright © 2005 by W. H. Freeman & Company
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 !!!
Gravitational energy of contracting gas to thermal energy is “KelvinHelmholtz contraction”
Proto planetary disk
• 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