Pluto was demoted and then there were 8. But why is the Kuiper Belt's mean orbit so tilted from the ecliptic? Some suggest a Planet 9 out there causing orbits to skew. Now some even suggest it might be a small planet mass Black Hole.
This is one of those stories that is unlikely but so incredibly interesting that is gets lots of attention. It also shows how ideas flow through the Physics community so a bit about the train of thought...
It was noted that of the few Kuiper belt objects detected, the average plane of orbit is tilted with regard to the normal solar system "ecliptic". Why? Well first remember that there have only been a few objects mapped so this isn't a lot of data and may even be an anomaly but since we do not know otherwise, this led Michael Brown and Konstantin Batygin to run simulations to see what might cause this. They concluded that it was very likely we had an extremely distant planet of something like 5-10 times the mass of the earth that revolved around the sun every 10,000 to 20,000 years. Why isn't such a heavy object visible? They reasoned that it must be dense and so much smaller than a neptune-like planet (the most remote known planet). Perhaps it was a rocky "super-earth", which is one of the most abundant planet types yet discovered.
On another thought track, recent simulations of the early universe after the Big Bang pointed to the formation of what are called "Primordial Black Holes." These, if they exist, would be incredibly ancient. The effects of Hawking Radiation escaping from these Black Holes for around 16 billion years might well cause them to be in the same mass range as the hypothesized Planet 9. Since no other method for creating such small Black Holes is known, the primordial nature is important to this train of thought.
Put the two ideas together and it is possible that Planet 9 is so dark because it is actually a very tiny object (about the size of an orange) that emits no light – an evaporation diminished Primordial Black Hole. There is also no reason a Black Hole of this size captured by the sun's gravity would orbit the sun in its ecliptic so it fits the Planet 9 hypothesis well.
So how do we detect if a tiny Black Hole is out there in the Kuiper Belt? Well if it is out there we have a new observatory that could see comets being hoovered up and it should only take a year or so to disallow this explanation.
To put a final spin on this discussion I like what Dr. Brown said after conceding that it was possible Planet 9 was a Black Hole:
“But it doesn’t make sense,” he said. “It is also possible that Planet 9 is a six-Earth-mass hamburger, I guess.” NYTimes.com
What? A massive hamburger out in the Kuiper Belt? Not sure which would be more surprising.
- Is There a Black Hole in Our Backyard? New York Times https://www.nytimes.com/2020/09/11/science/astronomy-planet-nine-black-hole.html?action=click&module=Top Stories&pgtype=Homepage
- What if Planet 9 is a Primordial Black Hole? Arxiv.org https://arxiv.org/abs/1909.11090
- Planet X? Why not a tiny black hole instead? Ars Technica https://arstechnica.com/science/2020/08/planet-x-why-not-a-tiny-black-hole-instead/
- Could theorized Planet 9 be a primordial black hole? Researchers propose method to find out. Phys.org https://phys.org/news/2020-05-theorized-planet-primordial-black-hole.html
- Planet Nine could be a primordial black hole, new research suggests. Phys.org https://phys.org/news/2019-09-planet-primodial-black-hole.html
- The Search for Planet 9 | Dr. Renu Malhotra | TEDxPortland https://www.youtube.com/watch?v=MptrypvBTag
- Does Planet 9 Exist? Veritasium https://www.youtube.com/watch?v=pe83T9hISoY