The outer solar system is pretty well accounted for. For many years we thought it wasn't, and kept hunting for another (sufficiently-massive so not Pluto) plan

The outer solar system is pretty well accounted for. For many years we thought it wasn't, and kept hunting for another (sufficiently-massive so not Pluto) planet beyond Neptune. The discrepancy turned out to be an overestimation of Neptune's mass, which was resolved by Voyager and subsequent observations. We've now ruled out anything of Neptune's mass or greater existing out to at least 100 AU and likely several multiples of that.