Try this movie of jupiter as seen from one of its moons.

They are far away:

• Jupiter: 5 AU
• Saturn: 10 AU
• Uranus: 19 AU
• Neptune: 30 AU

They are big:

• Jupiter: 11 R_E
• Saturn: 9 R_E
• Uranus: 4 R_E
• Neptune: 4 R_E

Some of them rotate fast:

• Jupiter: 10 hours
• Saturn: 10 hours
• Uranus: 17 hours; axis tilted, approximately in plane of orbit.
• Neptune: 16 hours

Their densities are low:

• Jupiter: 1300 kg/m^3
• Saturn: 700 kg/m^3
• Uranus: 1300 kg/m^3
• Neptune: 1600 kg/m^3

The low densities suggest that these planets are mostly made of gas. They are cold at their cloud tops:

• Jupiter: 160 K
• Saturn: 90 K
• Uranus: 60 K
• Neptune: 60 K

Low temperature at start of solar system would allow

• ices to form
  • water, H_2O
  • methane, CH_4
  • ammonia, NH_3
  • carbon dioxide, CO_2
• gasses to be attracted by gravity to a central core planet more easily.
  • This fits with the low density.
  • Available gasses are H and He.
• And of course "rock" is also available.

• What can we say about the structure of the jovian planets?
• Why does Jupiter have those bands?