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When a star is born, excess matter condenses into
proto-planets which are captured in the gravity of the newly born
star. When the Sun was born, many such proto-planets circled around
it, colliding with one another to form full size planets.
Most scientists believe that one such proto-planet
collided with the fledgling Earth at some point in its early development.
Most of the body which struck Earth was vaporized by the impact.
A large portion which survived combined with parts of Earth's
crust, and was captured by Earth's gravity and became our moon.
This theory is called the Giant Impact Theory, and is the most
widely accepted theory on the formation of our moon.
Other scientists think that the moon may have been
created elsewhere in the solar system, and was merely captured
by the Earth's gravitational pull as it passed by. This theory
is not widely accepted, as it is unusual that the moon would have
ended up in its current orbit if it were captured this way.
Another theory is that the moon and the earth were
formed out of the same material, and both were created at the
same time, one orbiting the other. Once again, this theory has
few proponents, because the composition of the moon does not match
that of the Earth.
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