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| The theory that the planets formed from a spinning disk of material around the forming sun. |
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| Small, rocky world. Most asteroids orbit between Mars and Jupiter in the asteroid belt. |
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| One of the small, icy bodies that orbit the sun and produce tails of gas and dust when they approach the sun. |
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| An Earthlike planet - small, dense, rocky. |
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| Jupiter-like planet with a large diameter and low density. |
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| The four largest satellites of Jupiter, named after their discoverer, Gailileo. |
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| The collection of icy planetesimals orbiting in a region from just beyond Neptune out to 50 AU or more. |
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| A small bit of matter heated by friction in incandescent vapor as it falls into Earth's atmosphere. |
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| A meteor in space before it enters Earth's atmosphere. |
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| A meteor that survives its passage through the atmosphere and strikes the ground. |
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| Stony meteorite that contains small glassy spheres called chondrules and volatiles. These chondrites may be the least-altered remains of the solar nebula still present in the solar system. |
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| A display of meteors that appear to come from one point in the sky, understood to be cometary debris. |
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| The time required for half of the radioactive atoms in a sample to decay. |
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| The density a planet would have if its gravity did not compress it. |
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| Boundary beyond which water vapor could freeze to form ice. |
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| The sequence in which different materials condense from the solar nebula depending on distance from the sun. |
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| One of the small bodies that formed from the solar nebula and eventually grew into protoplanets. |
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| The growth of a particle by addition of material from surrounding gas, atom by atom. |
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| The sticking together of solid particles to produce a larger particle. |
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| Massive object, destined to become a planet, resulting from the coalescence of planetesimals in the solar nebula. |
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| The process by which a forming body such as a planet gravitationally captures gas rapidly from the surrounding nebula. |
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| The separation of planetary material inside a planet according to density. |
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| The release of gases from a planet's interior. |
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| The intense cratering that occurred sometime during the first 0.5 billion years in the history of the solar system. |
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| A small solar system body (asteroid or comet) within an orbit near enough to Earth that it poses some threat of eventual collision. |
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| An explanation of a phenomenon involving slow, steady processes of the sort seen happening in the present day. |
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| An explanation of a phenomenon involving special, sudden, perhaps violent, events. |
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| In planetology, the heat released by infalling matter during the formation of a planetary body. |
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| The hypothetical source of comets, a swarm of icy bodies understood to lie in a spherical shell extending to 100,000 AU from the sun. |
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| A disk of dust found by infrared observations around some stars. The dust is debris from collisions among asteroids, comets, and Kuiper belt objects. |
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| A planet orbiting a star other than the sun. |
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