Planet 9

Evidence found by CalTech mathematicians suggests that there may be a ninth planet deep in the solar system. The research was published in January 2015 by CalTech astronomers  Konstantin Batygin and Mike Brown. Batygin and Brown have nicknamed the theoretical planet “Planet Nine,” however official naming rights are given to the people who actually discover the planet, so it is temporarily being referred to as “Planet X.” 

While this is just a theory, it could explain abnormal orbit patterns of smaller objects in the Kuiper Belt. The Kuiper Belt is a region far beyond the orbit of Neptune filled with icy debris. Astronomers who study the Kuiper Belt have noticed that dwarf planets and other debris follow orbits that cluster together, and the study of these abnormal orbits led some astronomers to hypothesize an additional planet beyond Pluto. Considering their estimations, the gravity of the potential planet could describe these unique orbits in the Kuiper Belt.

Planet X is believed to be large. It is speculated to be around the same size as Neptune or Uranus and 10 times the mass of Earth. The planet is also hypothesized to have an extremely extended orbit. It would be around 20 times farther from the sun compared to Neptune, and one complete revolution would take around 10,000 to 20,000 years. For reference, Neptune's orbit takes around 165 years to complete, and Earth’s is one year. 

This theory is based on modeling and simulations, rather than observation. The next step is exploration - searching for the planet. Astronomers, including Batygin and Brown, will use tremendously powerful telescopes to try and spot Planet X. 

References:

 NASA. (n.d.-b). Hypothetical planet X. NASA. https://science.nasa.gov/solar-system/planet-x/