Planet Nine is a theoretical object proposed to explain the unusual elongated orbital paths of certain remote trans-Neptunian objects (TNOs) in the outer Solar System. These anomalous orbits suggest the presence of a massive body, estimated to be about 5 to 10 times the mass of Earth, influencing the TNOs from a distance of hundreds of AU. (1AU = 150million km or 93million miles).

Traditionally, astronomers have assumed Planet Nine is either a gas giant planet or a “super-Earth” that either formed with the Solar System or was captured from interstellar space. However, despite extensive searches, no direct observation of such a planet has ever been made. This has led researchers to explore alternative explanations, including the concept that Planet Nine could be a small primordial black hole instead.