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