A new theory of how the Moon was formed, published in Scientific American:

Earth collided with another planet, vaporizing the outer layers and forming a continuous body of material called a synestia.This likely happened several times in Earth's history, but in one case, as it condensed, the Moon was formed.Why it matters, per author Simon Lock: "The discovery of synestias is a game changer for our understanding of how the Earth formed. It is likely that several times during formation, the body that would become the Earth was transformed into an extended, substantially vaporized body. This dictates how the core of the Earth formed, how we acquired our ocean and atmosphere, and maybe how our planetary companion, the Moon, came into being."

Current theory: A Mars-sized planet called Theia slowly bumped up against Earth, knocking off material that formed into a disk of molten rock around our planet. But more recently, researchers found that the speed of rotation for the Earth and Moon could have been much greater when they first formed and scientists began to study how else the Moon may have been created. Lock says synestias come in many shapes and sizes and could change our understanding of how planet formation beyond our solar system.