The term "Ninth Planet" refers to a hypothetical celestial body in our solar system, often called Planet Nine. It is theorized to exist beyond the orbit of Neptune and is believed to be influencing the orbits of distant objects in the Kuiper Belt. Astronomers suggest that this planet could be about ten times the mass of Earth and may take thousands of years to complete one orbit around the Sun. While no direct observations have confirmed its existence, ongoing research and sky surveys aim to gather more evidence to support or refute the theory of a Ninth Planet.