Planetary system

A planetary system consists of at least one star and various orbiting objects (such as asteroids, comets, moons, and planets). Our own planetary system which contains the Earth and Sun is called the Solar System.

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Origin and evolution of planetary systems
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 which surrounds pulsars.

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Significant planetary systems
Solar System - our planetary system, the first planetary system discovered, the first multiplanet planetary system discovered
PSR 1257+12 - the first extrasolar planetary system discovered, the first pulsar planetary system discovered, the first multi exoplanet system discovered, the first multi planet system with a pulsar discovered
Upsilon Andromedae - the first multiplanet extrasolar planetary system discovered around a main sequence star, found to be so in April 1999
PSR B1620-26 - the first multistar planetary system discovered
55 Cancri - the largest extra solar planetary system discovered (4 planets, as of August 2004)
Gliese 876 - the first system around a red dwarf star and the first discovered to be in an orbital resonance

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