Planet X has a long and storied history of nonexistence. For about 130 years, astronomers have been discussing the existence of one or more additional planets in order to explain discrepancies in the orbits of the known planets (mainly Neptune and Uranus). The list of discrepancies was later expanded to cover Trans-Neptunian objects. But none of the Planet X candidates discovered, including Pluto, has the mass or location to explain observations.
Original black holes have now been proposed as the newest Planet X (or Planet 9 since Pluto was downgraded).
The origin of Planet X begins with the discovery of Neptune. Neptune wasn't found by chance: observations of curiosities in Uranus' orbit were used to calculate the location of Neptune, and it was subsequently found.
This is a game that can be played more than once. The astronomers then found that the orbits of Uranus and Neptune could be better explained by the existence of another large planet. Follow-up observations have found numerous objects: Pluto and Charon, Sedna and Eris, to name a few. None of these distant bodies are big enough to be Planet X. However, some of their orbits could also suggest that a new planet is needed.
Some Trans-Neptunian objects have very strange orbits. Many are grouped and have highly elliptical orbits, and there is a subgroup that orbits far beyond the plane in which the planets orbit. This is unusual because the effects of gravity and the type of disk that formed the planets usually keep everything close to the same orbital plane. Finding sharply sloped orbits suggests that something is pulling the objects back from the aircraft.
A planet large and far enough from the sun could explain these orbits. In fact, researchers have calculated a number of different planetary masses and orbits that can explain the behavior of objects like Sedna.
By lucky coincidence, the required masses and distances correspond to an observed anomaly of the gravitational lenses – an excess of events in which the gravitational influence of an invisible object distorted the light of distant stars. That is, there is mass out there and it is invisible. Could this be a new planet?
A series of unlikely events
So there seems to be mass, but where does it come from? The planet would have to be about 300-1,000 AU from the Sun (for reference, Neptune's orbit is only 30 AU), where there is very little rock or gas. Simply put, a planet couldn't form out there. One possibility is that the planet formed closer to the sun and was ping-ponged there through interactions with one or more of the gas giants. However, to stabilize in a distant orbit requires a passing star (or something similar), which seems unlikely. The remaining option is that our sun caught a freely wandering planet.
But hikers must have been evicted from their own solar system, which means most of them move around with a fair clip. Therefore, the chance of capturing such a planet in the required orbit is slim, if not impossible.
The new paper argues that if we are looking at a low probability event, why not a primordial black hole? Original black holes could have formed shortly after the Big Bang. And, unlike black holes formed from collapsing stars, they could have masses as small as 10 µg and up. That said, there should be some with the correct mass range. How many is a matter of speculation.
Attracted by the idea, the researchers started rolling d20s: they set original black holes to an arbitrarily low number and then concluded that trapping a black hole is about as likely as trapping a wandering planet.
For a cent, for a pound
If Planet X were indeed a black hole, how would we know? The researchers argue that the annihilation of dark matter is the thing to be looked for.
Nobody knows whether primitive black holes exist. Nobody knows if dark matter annihilates, and if so, there is no certainty that it will happen in a discernible way – dark matter could annihilate itself to create other forms of dark matter that leave us in the dark. A speculative product of the early universe can therefore only be directly proven if another speculative process is carried out in the right way. In this case, it can be confirmed whether an original black hole is part of our solar system family.
Underlying the speculation is an interesting coincidence: unexplained gravitational lens events that happen to have the correct mass and distance to explain some very strange orbits of Trans-Neptunian objects. That coincidence seems to call for a single explanation of what the researchers are trying to do. That makes your house of cards useful.
Physical Review Letters, 2020, DOI: 10.1103 / PhysRevLett.125.051103 (Via DOIs)