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Learn about planets outside our solar system through Exoplanets and Alien Solar Systems by Tahir Yaqoob, Ph.D., a book in our online store book collection.
This is a composite image of the small moons of Saturn.
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NASA

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 is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA