At the cold edge of our solar system, astronomers are following the invisible thread of gravity toward something that has never been seen — a hypothetical ninth planet whose presence is written not in light, but in the subtle, persistent bending of other worlds' paths. Since 2016, the unusual orbital clustering of distant icy bodies in the Kuiper Belt has resisted explanation by known forces alone, inviting the possibility that a massive, unseen ice giant moves through the outer dark. The search is less a leap of imagination than a disciplined act of mathematical reasoning, one that could, if