In the past two decades, the roster of known planets in the galaxy has mushroomed. Astronomers have added to the handful in our own solar system roughly 450 so-called exoplanets orbiting other stars. Most of those planets are more massive than Saturn, which makes them unpromising from a habitability standpoint—such giants tend to be gaseous bodies without a surface to walk on.

But the giant planets in our solar system—Jupiter, Saturn, Neptune and Uranus—all have moons, some with planetlike features such as atmospheres, magnetic fields or active volcanoes….