The Planetary Systems Family
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Because of their mechanism of formation, planets tend to exist in systems, each bound to its central star. Although “free floaters” are known to roam interstellar space, in general planets as we know them live a social rather than a solitary life, mutually interacting not only gravitationally but also when material is exchanged between them, as in the case of the famous Mars rock ALH84001. In common usage, a planetary system consists not only of the planets themselves but also everything associated with them—what we term in this book the entire Kingdom of the Planets. At the Class level in this entry, we consider the planets as a system of objects orbiting a star. This is in contrast to “clustering,” as stars and galaxies tend to do under the action of gravitation. Until recently, there were no known analog planet “clusters” in the absence of a parent star, but in 2018 the Hubble Space Telescope did report one such binary planet with no star. For most of our history, our Solar System was the only planetary system known. Only in the last quarter-century have thousands of exoplanets been discovered around other stars, hundreds of them known to exist in multiple planet systems.