The inner planets are rocky and have diameters of less than 13,000 kilometers. The outer planets include Jupiter, Saturn, Uranus, and Neptune. The smaller, inner planets include Mercury, Venus, Earth, and Mars. Inner planet's atmosphere is thin. (Mercury has no atmosphere). Outer Planets: Outer planets' atmosphere is very thick. The four inner planets, Mercury, Venus, Earth, and Mars, are warmer than the outer gas giants. However, the temperature of the planets does not follow a linear path from the Sun.
