tranthuongbn
Posts : 45 Join date : 2010-12-29
| Subject: Planetary system Sun Jan 16, 2011 3:29 am | |
| Planetary systems 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, 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 a protoplanetary disk forms by gravitational collapse of a molecular cloud and then evolves into a planetary system by collisions and gravitational capture.[5] Some planetary systems may form differently, however. Planets orbiting pulsars—stars which emit periodic bursts of electromagnetic radiation—have been discovered by the slight variations they cause in the timing of these bursts. Pulsars are formed in violent supernova explosions, and a normal planetary system could not possibly survive such a blast—planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, 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.[6] Get rid of split ends4Life Transfer Factor Tri-Factor Formula | |
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