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| a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets |
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| a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail |
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| gravitational separation of materials of different density into layers in the interior of a planet or moon |
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| any of the planets Jupiter, Saturn, Uranus, and Neptune in our solar system, or planets of roughly that mass and composition in other planetary systems |
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| time required for half of the radioactive atoms in a sample to disintegrate |
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| a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light |
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| a portion of a meteor that survives passage through an atmosphere and strikes the ground |
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| objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary objects we see today; the comets and some asteroids may be leftover planetesimals |
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| process by which certain kinds of atomic nuclei decay naturally, with the spontaneous emission of subatomic particles and gamma rays |
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| the cloud of gas and dust from which the solar system formed |
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| any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list |
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