OK, so humans have been watching the sky for millennia, and we have discovered a lot of objects orbiting the Sun, we've learned a lot about our own galaxy, the Milky Way; and we have learned a lot about how we are not the only galaxy in the universe, but one of more than a trillion of galaxies flying away from each other in all directions through the vastness of space and time! Because there are a mind-numbing number of objects out there in space-time, we still do not have a solid handle on our own neighborhood, our solar system. For many years, some scientists have wondered and even expected to find "Planet 9" or "Planet X", another planet well beyond the orbits of Neptune and the former planet Pluto. There are reasons for this suspicion, of course, but no direct evidence for it.
But now, there is a new paper that provides the BEST statistical analysis and reasoning yet that requires the existence of Planet 9, which is needed to explain small perturbations in the orbits of the outer planets. This is why science is so cool! There will always be new information, measurements, observations, and theories coming in and out of existence as scientists try to piece together countless objects moving around the universe, all of which influence each other through gravity. It is ridiculously complicated and hard, but that's where the fun is for us, trying to figure it out! The diagram is via Wikipedia.
