The origin of the Earth and Moon is one of science’s greatest mystery stories, complete with false starts and dead ends. The Apollo missions shattered all the previous ideas about making the Moon. But the precious lunar samples contain a major clue to our planet’s creation: the Moon is Earth’s isotopic twin. The isotopes of different elements are like a planetary fingerprint: no two bodies are the same – except the Earth and Moon. After Apollo, a giant impact became the most likely explanation for the Moon, but it failed to explain this key observation. Stewart will explain how the accidental discovery of a new type of astronomical object, called a synestia, may save the idea of a giant impact and forever change the way we think about the birth of our planet.