Planet Nine is a hypothesized massive planet, first proposed in 2014, that is speculated to orbit far out in the solar system. It has not been detected formally. Instead, astronomers have inferred its presence from perceived perturbations of the orbits of some Trans-Neptunian Objects (TNOs). The planet is estimated to be 2 to 4 times the radius, and about ten times the mass of the Earth. (Image credit: Mark Garlick/Science Photo Library/Getty Images) The best candidate yet for the elusive Planet...