“There have only been two true planets discovered since ancient times, and this would be a third. It’s a pretty substantial chunk of our solar system that’s still out there to be found, which is pretty exciting.” -Mike Brown, on the possibility of Planet Nine
When most people think of the Kuiper belt, they think of a population of objects just beyond Neptune, with slightly larger, more elliptical and more inclined orbits. As new discoveries like the recent 2015 RR245 show, however, there are a great many additional objects in the scattered disk with different orbital parameters that are much harder to find.
These objects are quite likely to exist in great numbers, and are very difficult to find with current technology. Our observational biases may have strong and profound implications for our Solar System, including for the potential existence or non-existence of the hypothetical Planet Nine.