by Jack J. Shamama
Pluto is 1/6th the size of Earth, but it has 5 moons. Jupiter has 67. Why does the Earth have just one moon?
One way of categorizing the planets of our solar system is to divide them between terrestrial and jovian. The terrestrial planets, Mars, Earth, Venus, and Mercury only have three moons between them (Mars has two, Phobos and Deimos, and the Earth has one). The Jovian planets, by contrast, are teeming with moons: At latest count, Jupiter has 67, Saturn has over 60, Uranus has 27, and Neptune has 14. As NASA's New Horizons probe approaches Pluto, it's counting five moons in orbit around the dwarf planet.
