Outer planets (Jupiter, Saturn, Uranus, Neptune) are all larger in mass. Inner planets (Mercury, Venus, Earth, Mars) are smaller in mass. The giant outer planets have extremely thick atmospheres of hydrogen, helium, methane, and ammonia. Where as the inner planets don not. Only two inner planets have a few moons in orbit but the outer planets have many. Only Earth and Mars have moons orbiting them. One moon orbits Earth and two small moons orbit Mars. All of the Outer Planets have many moons orbiting them. There are 63 moons known to orbit Jupiter, 60 orbiting Saturn, 27 orbiting Uranus and 13 orbiting Neptune.
