Many of the moons surrounding other planets are really
captured and not objects that formed in place with
the mother planet, or were objected by the mother planet.
One sign that a moon is really a captured asteroid is that it
has a non-spherical shape, or looks more like a potato than
round like a Moon. Moons that look like this are the two
moons of Mars; Phobus and Deimos. Another sign that a
moon may be captured is if it orbits in a direction opposite
to that of the mother planet. An example of this
kind is Neptune's moon Triton.
The Earth's Moon is both rounded in shape and orbits with
Earth. These are the most direct pieces of evidence that the
Moon is not a captured object. If the Moon were a captured
object it would explain why the Moon and the Earth do not
appear to be made of the same material.