In one of the wilder theories attempting to explain the orbits of the outer objects found beyond Neptune, two physicists have proposed that the reason Planet X has not been located is because it might be a small black hole.

Previous studies have suggested Planet Nine, which some astronomers refer to as “Planet X,” has a mass between five and 15 times that of Earth and lies between 45 billion and 150 billion kilometers from the sun. At such a distance, an object would receive very little light from the sun, making it hard to see with telescopes.

To detect objects of that mass, whether planets or black holes, astronomers can look for weird blobs of light formed when light “bends” around the object’s gravitational field on its journey through the galaxy (simulated image above). Those anomalies would come and go as objects move in front of a distant star and continue in their orbit.

But if the object is a planet-mass black hole, the physicists say, it would likely be surrounded by a halo of dark matter that could stretch up to 1 billion kilometers on every side. And interactions between dark matter particles in that halo—especially collisions between dark matter and dark antimatter—could release a flash of gamma rays that would betray the object’s presence, the researchers propose in a forthcoming paper posted on the preprint server arXiv.

Anything is possible, but some things are certainly less likely than others. If these scientists turn out to be right, however, they will have achieved one of the biggest coups in the history of science.

And yes, the undiscovered planet out there should be referred to as “Planet X”, not “Planet Nine.” Not only is Pluto a planet, so are a lot of other objects in the solar system that up to recently were not considered so.