Beyond the 8 planets of our solar system, there is a massive expanse of inhospitable space. There have always been questions if there are any more planets, bigger than the icy world of the dwarf planet Pluto, but now some people are beginning to think that there is something much, much larger out there. Something that is several times the mass of earth named 'Planet Nine'. “Planet Nine” is thought to be a super-Earth, around 15 times larger than Earth. It was first theorized in 2016. The existence of this planet would explain why objects in the Kuiper Belt are clustered and aligned in a particular way. The Kuiper Belt is a doughnut-shaped region in the outer solar system beyond the orbit of Neptune. It is hoped to a ring of icy bodies, most famously Pluto, but also Haumea and Makemake.

The Solar System and their positions. Credit: Shutterstock

The hypothetical world of Planet Nine shows itself by the way its gravity has aligned the orbits of a group of these small, icy bodies. No one can imagine how a planet big enough to do that could form so far from the sun as. “All we know is that there’s an object of a certain mass out there,” Jakub Scholtz, a theorist at Durham University stated. “The observations we have can’t tell us what that object is.” He suspects that there could maybe be something even more exotic: a primordial black hole, that was forged in the big bang. If there is a currently undetected planet in the Kuiper Belt, it’s cold, dark, and way too small to see. However, if he is right, this would be a groundbreaking discovery. Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led to dense regions that could undergo gravitational collapse, forming black holes. They may even comprise dark matter which would further the universe as we know it.

Black hole at the center of galaxy M87, outlined by emission from hot gas. Credits: Event Horizon Telescope collaboration et al.

Theories that 'Planet Nine' couldn’t actually be a planet was fuelled by the theoretical problems involved in forming a large planet that far from our star. The planets of the solar system came from a disc of matter surrounding the sun. The matter thins out the further you get from the sun, meaning that there may not have been enough matter for a planet 15 times the size of Earth. At the distance Planet, Nine would be sitting, there simply isn’t enough raw material to build something that large. There is actually a hybrid scenario in which 'Planet Nine' formed much closer to the sun and was then catapulted into the expanse of the outer Kuiper Belt by the gravity of Jupiter or Saturn. But that becomes problematic because a single interaction can’t do the job. Instead, a string of interactions is needed to make sure Planet Nine never returns to where it was originally formed. For a lot of physicists, that theory seems to be a lot of coincidences that are statistically almost impossible. The Optical Gravitational Lensing Experiment (OGLE), at the Las Campanas Observatory in Chile, watches stars in the middle of the Milky Way for increases in brightness caused by gravitational microlensing. Gravitational microlensing is when light from background sources is bent by the passage of intervening objects that would otherwise be too small or too faint to be seen. They are revealed as their orbits temporarily line up between Earth and a star in the galactic centre such that their gravity focuses the light, making the star appear to brighten. The shorter the time span of this time of increased luminosity, the lower the mass of the intervening object.

OGLE project. Credit: University of Warsaw

In 2019, Scholtz and James Unwin, a physicist at the University of Illinois, put out a paper titled "What if Planet 9 is a Primordial Black Hole?". It revealed that such a black hole would be just 9 centimetres in diameter, roughly the size of a bell pepper. It went into detail as to why it is plausible, and it all comes down to how Planet Nine would get to its orbit If Planet Nine didn’t form in our solar system, the only other way to get it there is for it to be captured by the gravitational strength of our solar system whilst it was free-floating around. Different groups have come up with estimates, each showing that this is unlikely, but not impossible this is. In their paper, Scholtz and Unwin demonstrated that it was no less improbable to capture a primordial black hole. Unfortunately, optical telescopes will never be able to see a black hole. X-ray telescopes do stand a chance because anything falling into a black hole would heat up and give off X-rays. However, these flashes would be very fast, so we would have to be looking in exactly the right direction at exactly the right time to spot one. There is one other possibility that would give a steady X-ray signal, and that is if dark matter is made up of exotic particles that annihilate each other on contact; dark matter would tend to cluster around the black hole. As a result of the annihilation, it would emit a steady beam of X-rays or gamma rays that would drift across the sky as the black hole followed its orbit.

Rubin Observatory. Credit: Cerro Pachon Camera

For the moment, we need to continue charting the small, icy worlds of the outer solar system to determine whether Planet Nine or something else, is really out there. That will soon become easier thanks to the Vera C. Rubin Observatory in Chile, due to start operating this year. It is expected to find tens of thousands of mini-worlds in the remote reaches of the solar system, vastly increasing the current size. Their orbits will prove once and for all whether there is a planetary-mass object out there aligning the orbits or if there is noting actually there. If the Rubin Observatory sees a planet, it will be a huge discovery. If it doesn’t see anything and yet the anomalous gravitational pull remains, it will be time to see if the Black whole really exists. Thank you for reading this blog, let me know what you think!

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