'Nibiru', also known as Planet X, is crashing through our solar system at a doomsday-causing rate. Internet conspiracists are certain that the world is going to end.
Nibiru, as the legends go, is between four and ten times as big as Earth, depending on your conspiracist digest of choice, and it has an orbit far greater than Pluto, so that every couple of hundred thousand years it returns, making a fly by somewhere between Neptune and Mars, causing a big ol' regular extinction on Earth.
It was first chattered about on conspiracy website ZetaTalk, a website set up by Nancy Lieder from Wisconsin, to talk about her interaction with aliens.
The end of times due to this galactic bull-in-a-china-shop has been predicted in 2003, 2007, 2012 and 2015, and as of yet it hasn't come to fruition. This is mostly because actual scientists (the ones who are making studies, not poorly edited YouTube videos) point out Nibiru is not real.
An object that large orbiting to close to Earth would be visible to the naked eye and would cause massive changes to the orbits of other planets. Conspiracy theorists say that the object could be concealed by the sun, but as has been pointed out, this is geometrically imposisble.
Caltech Astronomer Mike Brown also noted that if the object existed on the suggested orbit, Jupiter would have expelled it already.
Brown has said, however, that he and fellow scientists may have found evidence of a large planet in distant orbit around the Sun - distant being the key word.
Hey, so, fun fact? Planet Nine is not going to cause the earth's destruction. If you read that it will, you have discovered idiotic writing!