Planets do sometimes collide with each-other,
although they cannot "break out" of gravity
(gravity extends throughout the entire visible
universe). Earth's moon probably formed when the
Earth got hit by a planet about the size of Mars
early in its history. This would have destroyed
all life on Earth if there had been any at the
time. However, had there been any life on the real
Mars, or on Venus, at that time, they would
probably not have even noticed.
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