In recent decades the scientific consensus has been that the moon formed billions of years ago from debris cast off when a Mars-sized object dealt Earth a glancing blow. But a radical new theory holds that some long-ago giant collision actually disintegrated Earth, causing it to balloon out into a vast doughnut-shaped cloud of vaporized rock, which the scientists who developed the theory dubbed a “synestia." They say the moon subsequently formed within this cosmic maelstrom. In the long-accepted explanation of lunar formation, the collision between Earth and the Mars-sized object, known as Theia, ejected part of Earth's mantle —...