According to NASA, Planet X does not exist, so there are no facts about it. As of 2015, the space agency's Wide-Field Infrared Survey Explorer, or WISE, found no evidence of a large, undiscovered planet within the solar system or even on the fringes of the solar system. No stars were found there, either.
The WISE project was in operation between 2010 and 2011, with a brief hiatus. During that time it studied more than 750,000,000 asteroids, stars, galaxies and other bodies. About 762 of these objects had never been seen before. Another study discovered 3,525 brown dwarfs and stars, but none of them were in the solar system. Some very close stars included a binary system of two brown dwarfs that were 6.5 light years from Earth. Though this is close in astronomical terms, it is very far out of the solar system. It takes whatever light these brown dwarf stars emit 6.5 years to reach Earth.
The existence of Planet X was first posited by the astronomer Percival Lowell, who noticed that something was perturbing the orbits of Neptune and Uranus. He believed it could have been another gas giant such as Jupiter, but the object turned out to be the dwarf planet Pluto, which was discovered in 1930.