Chapter 20

    I. Overview of the solar system

       A. Solar system includes

             1. Sun

             2. Nine planets and their satellites

             3. Asteroids

             4. Comets

             5. Meteoroids

       B. A planet's orbit lies in an orbital plane

             1. Similar to a flat sheet of paper

             2. The orbital planes of the planets are inclined

                  a. Planes of seven planets lie within 3 degrees of the Sun's equator

                  b. Mercury's is inclined 7 degrees

                  c. Pluto's is inclined 17 degrees

       C. Two groups of planets occur in the solar system

             1. Terrestrial (Earth-like) planets

                  a. Mercury through Mars

                  b. Small, dense, rocky

                  c. Low escape velocities

             2. Jovian (Jupiter-like) planets

                  a. Jupiter through Neptune

                  b. Large, low density, gaseous

                  c. Massive

                  d. Thick atmospheres composed of

                      1. Hydrogen

                      2. Helium

                      3. Methane

                      4. Ammonia

                  e. High escape velocities

             3. Pluto not included in either group

       D. Planets are composed of

             1. Gases

                  a. Hydrogen

                  b. Helium

             2. Rocks

                  a. Silicate minerals

                  b. Metallic iron

             3. Ices

                  a. Ammonia (NH₃)

                  b. Methane (CH₄)

                  c. Carbon dioxide (CO₂)

                  d. Water (H₂O)

II. Evolution of the planets (see textbook Introduction)

       A. Nebular hypothesis

             1. Planets formed about 5 billion years ago

             2. Solar system condensed from a gaseous nebula

       B. As the planets formed, the materials that compose them separated

             1. Dense metallic elements (iron and nickel) sank toward their centers

             2. Lighter elements (silicate minerals, oxygen, hydrogen) migrated toward their surfaces

             3. Process called chemical differentiation

       C. Due to their surface gravities, Venus and Earth retained atmospheric gases

       D. Due to frigid temperatures, the Jovian planets contain a high percentage of ices