There are currently eight planets in the Solar System. However, there is evidence to suggest that there may be a ninth planet. This planet has been nicknamed “Planet 9”.
The existence of Planet 9 was first proposed in 2016 by astronomers Mike Brown and Konstantin Batygin. They suggested that Planet 9 may be responsible for the strange orbits of some trans-Neptunian objects (TNOs). TNOs are small bodies that orbit the Sun at distances greater than Neptune.
Brown and Batygin used computer simulations to show that the orbits of TNOs could be explained if there was an undiscovered planet orbiting at a distance of about 700 AU from the Sun. (One AU is the distance between Earth and the Sun.)
Since then, other astronomers have searched for Planet 9, but it has not yet been discovered. It is thought to be about 10 times the mass of Earth and have an orbit that takes between 10,000 and 20,000 years to complete.
If Planet 9 exists, it would help us to understand how our Solar System formed and evolved. It would also provide clues about the formation of other planetary systems around other stars.