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Five years ago two of us whiled away a cloudy night on the summit of Mauna Kea in Hawaii by placing bets on how many moons remained to be discovered in the solar system. Jewitt wagered $100 that a dedicated telescopic search could find, at most, 10 new ones. After all, he reasoned, in the entire 20th century, astronomers had come across only a few. Sheppard more optimistically predicted twice as many, given the increased sensitivity of modern astronomical facilities.
Sheppard is now a richer man. Since that night, our team has discovered 62 moons around the giant planets, with more in the pipeline. Other groups have found an additional 24. (In strict astronomical parlance, they are "satellites," not "moons." There is only one moon and it is Earth's satellite. But even astronomers generally adopt the popular usage.) No one predicted that the family of the sun had so many members lurking in the shadows. They are classified as "irregular," meaning that their orbits are large, highly elliptical and tilted with respect to the equators of their host planets. So-called regular moons, such as Earth's or the large Galilean satellites of Jupiter, have comparatively tight, circular and nearly equatorial orbits.