Planet 9 is a hypothesized large planet that would be located in the outer parts of our solar system, far beyond the orbits of Neptune and Uranus. The existence of planet 9 would explain the weird orbits of some transneptunian objects that are hard to be explained without it. Scientists calculated that the planet would have a mass of approximately five to ten times that of Earth.
Image Credit: Alain r via Wikimedia Commons - HDR tune by Universal-Sci
As evidence for the existence of planet 9 is growing, scientists deliberate on its origin. Caltech researchers Michael E. Brown and Konstantin Batygin suggested that it might be the core of a giant planet that was expelled from its initial orbit by Jupiter during the genesis of our solar system. Others asserted that we might have stolen planet 9 from another star system a long time ago.
The reason that astronomers have yet to spot the planet is that it has to be at an enormous distance from Earth. The planet is on a distant, elongated orbit, 400 to 500 times further away from the sun as Earth.
Recently astronomers published a paper containing an even more fascinating theory about planet 9. They postulate that the planet might even not be a planet at all but rather a primordial black hole the size of a tennis ball.
Primordial black holes are ancient black holes that formed shortly after the Big Bang. They are not born in a conventional way via the collapse of a giant star. In the early universe, high densities and inhomogeneous conditions could have led adequately dense regions to experience gravitational collapses, creating small black holes without the need for collapsing stars.
An artist’s impression of planet 9, showing the rest of our solar system in the far distance - Image Credit: nagualdesign; Tom Ruen / ESO via Wikimedia Commons
Up until recently, scientists assumed that tiny primordial black holes probably did not exist. However, by way of the Optical Gravitational Lensing Experiment, astronomers have spotted several puzzling, small objects about five times the mass of Earth within our galaxy. A plausible explanation for these observations could be the existence of tiny black holes. If these objects are in fact primordial black holes, the galaxy must be filled with a large amount of them. Having one in our solar system would then not be as extremely far fetched as before.
So what will we find at the edge of our solar system? A planet or a black hole? This is incredibly tough to determine as there is almost no difference between the gravitational effects of these two types of objects when they have the same mass.
So if Planet 9 is, in fact, a tiny black hole, will we ever be able to see it? Scholtz and Unwin, the authors of the aforementioned paper, stated that primordial black holes have a very unique trademark. It is thought that such black holes would be enclosed by a halo of dark matter. The destruction of dark-matter particles would create gamma rays which might be visible using specialized telescopes. So we might actually be able to spot it using the right equipment.
Regardless of it being a planet or a black hole, we can't wait to learn more about the intriguing mysteries of planet 9. Stay tuned!
Sources: Optical Gravitational Lensing Experiment / MIT Technology Review / Paper: What if Planet 9 is a Primordial Black Hole?
Further reading: More evidence that Planet 9 is actually out there
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