Scientists have long debated the existence of a mysterious "Planet Nine" in the outer solar system. First proposed in 2016 by Konstantin Batygin and Michael Brown, Planet Nine is believed to be about 10 times the mass of Earth, with an orbit far beyond Neptune, taking 10,000 to 20,000 years to orbit the Sun. The evidence for Planet Nine comes from strange orbits of objects in the Kuiper Belt, suggesting the presence of a massive, unseen planet.