The Solar System contains 18 or 19 natural satellites of planets that are large enough for self-gravity to make them round. (Why the uncertain number? Neptune’s moon Proteus is on the edge.) They are shown here to scale with each other. Two of them are larger than Mercury; seven are larger than Pluto and Eris. If they were not orbiting planets, many of these worlds would be called “planets,” and scientists who study them are called “planetary scientists.”

Montage by Emily Lakdawalla. The Moon: Gari Arrillaga. Other moons data: NASA/JPL. Processing by Ted Stryk, Gordan Ugarkovic, Emily Lakdawalla, and Jason Perry.