• Big Ball O' Gas: 71% Hydrogen, 27% Helium, 2% heavier elements
• Radius = 7x10⁵ km (~100 Earth radii)
• Mass = 2x10³⁰ kg (~300,000 Earth masses)
• Energy from nuclear reactions: hydrogen fusion into helium.

The Planets and their Satellites

• The Terrestrial Planets: Mercury, Venus, Earth, Mars
• innermost planets
• smaller, rocky bodies (densities 4.0 - 5.5 g/cm³)
• complex atmospheres: CO₂, H₂O, N₂
• The Jovian Planets: Jupiter, Saturn, Uranus, Neptune
• more distant
• large, gaseous bodies (densities 0.7 - 1.6 g/cm³)
• primordial atmospheres: H, He
• and Pluto
• oddball

• Small, rocky bodies (several thousands known)
• Most lie in a belt between Mars and Jupiter
• Up to 1000 km in size (most much smaller)
• Total mass is less than that of the Moon
• When small ones enter Earth's atmosphere, we get meteors
• When big ones enter, we get Hollywood movies!

• Small, icy bodies
• Tails form when they interact with the Sun
• Travel on highly elliptical orbits
• Live in very distant regions of the solar system
• Primordial objects