Pluto, “Ultima Thule,” and the Lords of the Dark Realm
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Beginning at about the orbit of Neptune drifts a family of objects that area relics of the most ancient times, fossils of early planetary formation preserved in a deep freeze (Fig. 9.1). The region is called the Kuiper Belt. This vast donut of material is twenty times as wide as the Asteroid Belt and two hundred times as massive. It extends from 30 AU (at about the orbit of Neptune) to 50 AU, far beyond our main family of planets. It is the place left behind, the dregs of the solar nebula that remained at the outskirts as the inner solar nebula warmed and condensed into the planets and moons we see today.