The map of our solar system has changed. For most of history, we viewed our neighborhood as a tidy, organized place. We thought it was a simple system with eight planets orbiting a star, surrounded by a clean void. We assumed there was a clear finish line where the solar system stopped and "deep space" began. The Kuiper Belt proves that this model is wrong. The solar system does not have a hard edge. It does not just end. Instead, it fades away into the galaxy. What we once thought was empty space is actually a massive, chaotic frontier. It...