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The Capture Theory

Many of the moons surrounding other planets are really captured asteroids and not objects that formed in place with the mother planet, or were ejected 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 our Moon. Moons that look like this are the two moons of Mars; Phobos 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 a moon 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.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF