That means that of the tens of thousands of meteorites in collections around the world, likely none are from comets, leaving a significant gap in our understanding of the Solar System. When the Solar System formed, more than 4.5 billion years ago, a disc of dust and debris was swirling around the Sun. Over time, this material clumped together, forming larger and larger bodies -- some so large they swept up everything else in their orbit, and became planets. Yet some debris avoided this fate and is still floating around today. Scientists traditionally classify these objects into two groups: comets and asteroids.