Planet X is getting close to Earth. The force of it passing will cause the Earth’s magnetic poles to shift; it will cause huge earthquakes, volcanoes, Tsunamis and hurricanes all over the planet. They say Planet X is the tenth planet in our solar system.
It is not really a planet, actually a brown dwarf; a small, failed star. They give it a size of 4x that of Earth, but a density of 20x(more likely estimates put it 2-4 times the mass of Jupiter).
It orbits our Sun and travels very quickly when it passes close to it, but slowly in between. This acceleration is caused by gravity and is exactly the sort of thing you see with comets – ever wondered why comets orbit our sun but only appear for a few days every few decades?