The formation of the Moon was a great scientific mystery for many years. It was once thought that Earth and the Moon might have formed simultaneously as two separate objects, bound together by their mutual gravitational pull. This was shown to be unlikely after scientists proved that the two objects have very different compositions. Another idea suggested that Earth’s Moon formed elsewhere, and was later captured into Earth’s orbit as it went by Earth’s gravitational influence. The major problem with this scenario is that Earth and the Moon are relatively close in size; gravitational capture is very, very unlikely, unless one object is many times larger than the other.

Within the past few decades, scientists have shown that the most likely scenario of how the Moon formed involves the collision of two planetary bodies. Billions of years ago, before life formed on Earth, a Mars-sized protoplanet slammed into Earth at an angle. Most of the material in the protoplanet fell into, and became part of, our planet; some material, however, was thrown out into space, and began to orbit Earth as a ring of dust and rock. Within weeks, a large portion of that ring of material coalesced to form the core of our Moon; over millions of years, the Moon settled into its present-day size and shape.