If there's another planet out there in our solar system, where could it be? That's a question that astronomers have been trying to answer for years, but they've been coming up empty-handed.
We've noticed strange tilts and variations in the orbits of our Solar System's outer planets that suggest something big is out there gravitationally-influencing the typical orbital rhythm. This has been blamed on a potential planet beyond Pluto that's being called "Planet 9," but astronomers are already looking for "Planet 10" despite never finding the latter.
This planet would be so far away from the Sun that it wouldn't glimmer with reflection, and this makes finding it hard. Astronomers even think that it could be similar in size and mass to Earth and Mars.
The theory arose after astronomers studied the orbits of objects in the Kuiper Belt. After finding that the objects were orbiting on stranger planes than usual, it became apparent that something must have been gravitationally-tugging on them to cause it.
Whether it's there or not is up for debate, but one thing for certain is that we need to get to the bottom of whether there are any other planets orbiting our Sun that we don't already know about.