What are the Reasons there could/could not be a Planet X?

The object, which the researchers have nicknamed "Planet Nine," could have a mass about 10 times that of Earth and orbit about 20 times farther from the sun on average than Neptune. It may take between 10,000 and 20,000 Earth years to make one full orbit around the sun. read more

There's no evidence for the existence of Planet X, despite a NASA space telescope’s best efforts to track it down. The hypothetical world that may or may not be orbiting the sun beyond the orbit of Pluto has inspired many a doomsday theory. read more

They could not have had a clue that this Planet X they sought might spell doom for the planet, and all of the human race. The X in Planet X did not mean the tenth planet. At the time it would have only been the ninth planet. The designation of X simply meant an unknown variable, something science believed existed but had no proof of. read more