Kuiper Cliff Mystery
Why Does The Kuiper Belt Suddenly End? We understand mathematically why the Kuiper Belt exists: it was modeled before it was observed. But the model predicts it will continue, instead it just stop (drops off dramatically) and has been called the Kuiper Cliff. There's theories as to why (like one or more un-found planets, scrubbing the region), but for now, it's still a puzzle.