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The
traditional answer to this question is that planets are formed
out of the remnants of star birth. When a star is still in the
initial stages of being created, the molecular cloud surrounding
it begins to rotate. As it rotates, the cloud will begin to condense
around areas where there is more matter. These are called ‘protoplanets’.
These protoplanets will continue to accumulate matter by gravitational
pull, which increases as they grow larger. When they get big enough,
their gravity will be strong enough to attract gaseous materials
and form an atmosphere; some planets, like Jupiter, are comprised
mostly of gasses attracted in this way. In many instances the
protoplanets themselves will collide, generating enough heat to
‘fuse’ them together. Once the protostar around which these protoplanets
form becomes a star proper, solar winds will blow away the remnants
of the cloud, leaving only the protoplanets that have formed,
which have enough mass to withstand the force of the solar winds.
The protoplanets formed in this way will become asteroids, comets,
moons and planets, depending on their size. Most will collide
with each other to form a small number of planets, and those which
don’t actually collide will become satellites of the larger planets.
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