What term denotes a hypothetical distant planet thought to orbit far beyond Neptune?

• A. Tenth Planet
• B. Planet X
• C. Nibiru
• D. Planet Nine

Answer: (A)

When we label a hypothetical planet that would orbit far beyond Neptune, the simplest way to capture both its existence and its place in the lineup is to call it the tenth planet. This term directly signals that, if another planet were found beyond the known ones, it would come after Neptune as the next in sequence. Historically, this straightforward sequential label was used to describe such a distant world before more specific names emerged, making it the best fit for describing a hypothetical body beyond Neptune. Other terms exist—Planet X is a general placeholder for an unknown distant planet, Nibiru is a myth, and Planet Nine is a modern proposal—but they don’t convey the idea of a position in the planetary lineup as clearly as “tenth planet” does.