In 2015 American astronomers found strong evidence that a ninth planet might exist in our solar system.
This planet has been referred to as Planet X or Planet Nine and is thought to be about five to 20 times larger than Earth.
No-one has actually seen the planet yet, but scientists believe that it almost definitely exists, because they can see other objects in space being affected by it.
Planet X has still not yet been discovered and until scientists actually see the planet, they can't confirm its existence for sure.
Now, physicists Jakub Scholtz and James Unwin have made the claim that Planet X is in fact not a planet at all and could instead be a tiny black hole, the size of a bowling ball.
Earlier research suggested that there is an unseen gravitational source in the outer solar system on a 'weird orbit'. But that object could be planet or a black hole.
One of the possible explanations for the origin of Planet 9 is that it was a free-floating planet (a planet that got kicked out of its original star system), which flew close to our solar system which led to it orbiting around the Sun.
Unwin and Scholtz argue that a black hole could have done the same thing.
The gravitational source was seen to be affecting rocky objects in space which circle around the planet Neptune.
How can scientists examine it?
Physicists are looking for alternative ideas to what this gravitational source might be and want to use a special telescope to find out more.
However, there are other telescopes whose sensors are sensitive to other frequencies of light such as infrared, ultraviolet, or even x-rays and gamma rays.
As a result they can form images of objects that would be invisible to our naked eyes.
Astronomers have built many such telescopes and it's possible using one could help figure out what the mysterious object is.
What is a primordial black hole?
The new theory suggests a black hole - known as a 'primordial black hole' - could have been left over from the earliest days of the universe, shortly after the Big Bang.
Typically we assume that black holes are formed by a collapse of an old star that has spent all of its fuel and is heavy enough to form black holes — these kinds of black holes are at least as massive as our Sun and much more massive than a planet.
Primordial black holes were formed very early during the formation of our Universe. Some parts of the young Universe were so dense they could directly collapse into a tiny black hole — these are primordial black holes.
Primordial black holes have never been seen before but these observations could be the first hints that they exist.
So the hunt is still on for the mysterious object that lurks in the outer solar system!