Scientists have long believed that our solar system might have a ninth planet orbiting well beyond Neptune and Pluto. Scientists hypothesize that the mysterious planet would be between five and 20 Earth masses and would orbit on an elliptical some hundreds or thousands of times more distant from the Sun than the Earth.
With the planet thought to orbit so far from Earth-based telescopes, it could be impossible to find with current telescopes. The challenge is that the planet is thought to be so dim, about a million times dimmer than Neptune, that it could hide in the light pollution from the Milky Way. Earth’s most potent observatory currently is the Subaru Telescope in Hawaii.
That massive telescope can view a field in the sky about the size of 4,000 full moons at once. Even with such a gigantic field of view, Planet Nine is very difficult to observe. Researchers note that if the orbit of the mysterious world is beyond the 1,000 AU limit of current telescopes, it could lie invisible for the next 1,000 years.