Before the discovery of Pluto in the 20th century, it had been theorized that a ninth planet, Planet X, existed beyond Neptune due to the gravitational clustering that could only be caused by a massive object. It was then believed that this planet was found in Pluto, but that never fully quantified the gravitational distortion until scientists at the California Institute of Technology presented evidence that a ninth planet truly does exist with an orbital period of 15,000 years.
The astronomers who published their discovery have calculated that there is “only a 0.007 percent chance, or about one in 15,000, that the clustering could be a coincidence.” Presently, Planet Nine remains hypothetical, but astronomers have calculated its orbit to be quite massive. If it does exist, the planet would likely be approximately 2–15 times the mass of Earth and orbit between 200 and 1,600 Astronomical Units (AU) from the Sun. An AU is 150,000,000 kilometers, which means that the planet could orbit as far from the Sun as 240,000,000,000 kilometers.