1. 23.1 The Solar System

2. Terrestrial Planet Any of the Earth-like planets, including Mercury, Venus, Mars, and Earth.

3. Jovian Planet The Jupiter-like planets; Jupiter, Saturn, Uranus, and Neptune; These planets have relatively low densities and are huge gas giants.

4. Nebula A cloud of gas and/or dust in space.

5. Planetestimal Small, irregularly shaped body formed by colliding matter.

6. Key Concept Size is the most obvious difference between the terrestrial and the Jovian planets.

7. Key Concept Density, Chemical makeup, and rate of rotation are other ways in which the two groups of planets differ.

8. Key Concept According to the nebular theory, the sun and planets formed from a rotating disk of dust and gases.