On October 19th, astronomers discovered an objectunlike any other we’d seen before: a massive point of light hurtling through the inner Solar System. Sure, that might sound like any old asteroid or comet that just happens to pass close to the Sun, but this one was special. Unlike all the others we’ve ever seen, which came from the asteroid belt, the Kuiper belt, or even far out from the Oort cloud, this one was completely different. In fact, astronomers concluded, it came from outside the Solar System entirely. It’s one thing to be told that this is the case, but it’s quite another to understand how we know it must be so. Let’s find out!