Rocks are not just an Earthly phenomenon. They make up the surfaces of some of the planets of the solar system and, in one form or another, those all of the many moons, innumerable comets, and other small objects that orbit out to great distances from the Sun. They likely also lie deep beneath the immensely thick fluid envelopes of the gas giants. ‘Rocks on other planets’ describes what scientists have discovered about the rocks of Mercury, Venus, Mars, our moon, the giant gas planets, and distant moons and planets. The early exploration of exoplanets suggests that an even wider variety of rock formations likely exists on star systems other than ours.