The formation of the Sun had a dramatic effect on the temperatures across the solar nebula, introducing a temperature range that stretched from about 2000K near the Sun to less than 50K at the outer regions. The heat in the inner Solar System only allowed materials with high condensation temperatures to remain solid. These particles eventually gathered to form the four terrestrial planets: Mercury, Venus, Earth and Mars.

A similar process formed the outer planets of the Solar System: Jupiter, Saturn, Uranus, and Neptune. Yet, they are different because icy materials such as frozen water, carbon dioxide and methane were also available. Consequently, these outer Jovian planets are much larger than the terrestrial planets. In addition these giant planets were able to enhance their atmospheres by capturing gas atoms moving more slowly due to the colder temperatures.

Each planet travels around the Sun in an elliptical orbit that is held in place by the gravitational attraction between the Sun and the planet. Some of the planets, including, of course, Earth, have moons orbiting them. Mars has just two moons in orbit around it, while Jupiter has 63 moons known to be orbiting it. Kepler's three laws of planetary motion define the motion of the planets around the Sun, and the movement of moons around their parent planet.

Table 1.1: Distance and Orbital Parameters for the Planets

Table 1.2: Observational Characteristics of the Planets

	Solar System
	Asteroids and Comets