The Planet Nine hypothesis
Claim in the list: Astronomers hypothesize that a ninth planet exists in the outskirts of the Solar System.
Verification: Yes, this is a scientific hypothesis, not a confirmed discovery. In 2016, Caltech researchers published evidence for a distant massive planet that could explain orbital patterns among some trans-Neptunian objects. NASA says Planet Nine has not been discovered and whether it exists remains debated among scientists. Observational selection effects and alternative explanations for orbital clustering complicate the inference.
Conclusion: Planet Nine is a hypothetical object under search; its existence is unconfirmed.