Planetary science is in the midst of a revolution. As recently as the early 1990s, “the planets” consisted of just nine famous objects in our solar system that every school kid learned to recognize by name and appearance. But then, advances in astronomical technology unleashed an explosion of new planetary discoveries on two fronts. One of these fronts involved a bewildering variety of planets discovered around other stars. In rapid succession, we learned about extrasolar pulsar planets, hot Jupiters, superEarths and more. And there is now a widespread scientific consensus that the 300...