Body, other than a natural satellite (moon), that orbits the Sun and that is, for practical purposes, smaller than the planet Mercury yet large enough for its own gravity to have rounded its shape substantially. The International Astronomical Union adopted this category of solar system bodies in August 2006, designating Pluto, the even more-remote object Eris, and the asteroid Ceres as the first members of the category. Unlike major planets, these three bodies are not massive enough and are in orbits too elliptical, too inclined, or both to have swept up most smaller nearby bodies by gravitational attraction; they thus failed to grow larger.
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A pluton in geology is an intrusive igneous rock body that crystallized from a magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies. In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically kilometers in dimension, without a tabular shape like those of dikes and sills. Batholiths commonly are aggregations of plutons. The most common rock types in plutons are granite, granodiorite, tonalite, and quartz diorite.
The term originated from Pluto, the ancient Roman god of the underworld. The use of the name and concept goes back to the beginnings of the science of geology in the late 1700s and the then hotly debated theories of Neptunism, Vulcanism and Plutonism regarding the origin of basalt.