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A planetary system consists of the various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and cosmic dust. The Sun and its planetary system, which includes Earth, is known as the Solar System.
Origin and evolution Planetary systems around sun-like stars are generally believed to form as part of the same process which results in star formation. Some early theories involved another star passing extremely close to the sun, and drawing material out from it which then coalesced to form the planets. However, the probability of such a near collision is now known to be far too low to make this a viable model. Accepted theories today argue that planetary systems form from a solar nebula. Some planetary systems are very unlike our own, however: planetary systems around pulsars have been inferred from slight variations in the period of the pulses of electromagnetic radiation. Pulsars are formed in violent supernova explosions, and a normal planetary system could not possibly survive such a blast - planets would either evaporate, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star. One theory is that existing stellar companions were almost entirely evaporated by the supernova blast, leaving behind planet-sized bodies. Alternatively, planets may somehow form in the accretion disk surrounding pulsars. List See also | ||||||||
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