A collision with a planet-sized body named Theia is believed to have formed our Moon.

When Earth was very young, it was not alone in its orbit. Scientists believe another planet, about the size of Mars, crashed right into us. This ancient planet was called Theia. The impact was so powerful that it blasted huge chunks of Earth's outer layer into space. These hot, rocky leftovers eventually clumped together to form the Moon we see in the sky tonight.