Just 30 years ago, if you had asked an astronomer if there were planets around other stars beyond the Sun, they couldn’t tell you for certain. Although all the theories about planet formation indicated that they should exist around many stars, if not most of them, we had no evidence of planets beyond the Solar System. So we did the most natural thing you can imagine: we assumed others were like our own, with rocky worlds in the inner portions and gas giants in the outer reaches. Over the ensuing decades, we began discovering that our assumptions were gravely mistaken: practically all stars have planets; worlds of all sizes could appear anywhere in a solar system; there were many planets even larger than Jupiter; and most worlds were larger than Earth but smaller than Neptune. Yet, despite all we’ve learned, there seem to be only three classes of planet out there: Terran worlds, Neptunian worlds, and Jovian worlds.