Four theories attempt to answer this question:
• In ancient times, the moon broke away from the Earth. Scientists doubt this because the Earth was probably not rotating fast enough to cast off an entire moon.
• The Earth and the moon formed through accretion. Scientists also doubt this because the moon's composition differs from the Earth's.
• The moon came from elsewhere and was caught in Earth's gravity. It's unlikely, however, that the moon passed the Earth in just the right way to avoid colliding and instead establish an orbit.
• The most popular theory is that two large planets collided. The smaller one ended up orbiting the larger one. Because neither one was solid yet, the iron core of the smaller planet was caught in Earth's gravity. This would explain the differences in composition.
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