Image: Artist's impression of Planet Nine as an ice giant eclipsing the central Milky Way, with the Sun in the distance. Neptune's orbit is shown as a small ellipse around the Sun.
announced that they found indirect evidence of a possible ninth planet in our solar system. This indirect evidence came from the orbits of six different objects in the Kuiper Belt. The orbits of these objects was a mystery, since it was astronomically unlikely that the orbits of these objects would be the way they are unless something is acting on them. Using mathematics and simulations, the researchers Konstantin Batygin and Michael E. Brown at Caltech placed a hypothetical planet far beyond the orbit of Neptune with a mass about ten times that of the Earth. The orbits of these KBOs soon made sense if this planet existed, and no other hypothesis has yet been able to explain these orbits quite as well. Recently, astronomers found another object that followed the predicted orbit if Planet Nine exists. Planet Nine, if it exists, orbits the Sun at an extraordinary distance. It would be around twenty times farther from the Sun than Neptune is. It's so far away that it would take it between 10,000 and 20,000 years to complete one orbit around the Sun. If it exists, its orbit will be highly elliptical. Models predict that its closest approach to the Sun would be about 200 AU (an AU, or astronomical unit, is equal to the distance from the Earth to the Sun), and its farthest approach would be around 1,200 AU away from the Sun. So it's really far away!
If it exists, Planet Nine likely did not form as far as it is. Rather, it likely formed much closer to the Sun and was ejected to the far reaches of the outer solar system due to a close encounter with Jupiter or Saturn. It's also possible that Planet Nine could be a rogue planet that was caught by the Sun's gravity. Astronomers believe that if Planet Nine exists, it's very likely going to be similar in composition to Uranus and Neptune. So we may still have one more planet in our solar system to visit.