For more than fifty years, the world's top scientists searched for the "missing" planet Vulcan, whose existence was mandated by Isaac Newton's theories of gravity. Countless hours were spent on the hunt for the elusive orb, and some of the era's most skilled astronomers even claimed to have found it. There was just one problem: It was never there. In this book, Thomas Levenson follows the visionary scientists who inhabit the story of the phantom planet, starting with Isaac Newton, who in 1687 provided an explanation for all matter in motion throughout the universe, leading to Urbain-Jean-Joseph Le Verrier, who almost two centuries later built on Newton's theories and discovered Neptune, becoming the most famous scientist in the world. Le Verrier attempted to surpass that triumph by predicting the existence of yet another planet in our solar system, Vulcan. It took Albert Einstein to discern that the mystery of the missing planet was a problem not of measurements or math but of Newton's theory of gravity itself. Einstein's general theory of relativity proved that Vulcan did not and could not exist, and that the search for it had merely been a quirk of operating under the wrong set of assumptions about the universe. Levenson tells the tale of how the "discovery" of Vulcan in the nineteenth century set the stage for Einstein's monumental breakthrough.