The catastrophic collision that forged the moon, and marked one of the most consequential events in Earth's early history, may have been triggered not by a distant interloper, but by a sibling world that grew up right next door, according to a new study.
About 4.5 billion years ago, a Mars-size world slammed into the young Earth with such tremendous force that it melted huge swaths of our planet's mantle and blasted a disk of molten debris into orbit. That wreckage eventually clumped together to form the moon we know today. Scientists have long favored this "giant impact" origin story, but where the long-lost world, nicknamed Theia, came from and what it was made of remain a mystery.
