Where did the theory of planet number 9 come from?

This theory defends the existence of a planet beyond Neptune, responsible for the alteration of the orbits of several trans-Neptunian objects that we have been discovering little by little, basically those shown in the image, plus a few more, because every little bit it is being discovered. adding some more, by transneptunian object, to understand objects basically formed by large ice, in fact, some of them are already candidates for dwarf planets.

As you can see, trans-Neptunian objects, potential dwarf planets, have been discovered, some of them have been discovered to have slightly altered orbits with respect to what would be normal within the orbit around the Sun, and this alteration is believed to be one of the theories, it is believed that it could be caused by the existence of a massive planet, a large planet much larger than the Earth, it would be Uranus type, Neptune type, something like that, an icy gas giant, that planet would be the cause of those alterations in the orbits of those objects.

There are other theories such as, for example, that these objects had been altered by the passage of a nearby star, which is not so impossible or that there is a primordial black hole orbiting the Sun. Primordial black holes are theoretical objects, we have not yet discovered none. and the characteristic they have is that they can have a very small mass, small compared to the mass of normal stellar black holes.

I am talking about the mass of a planet or a planet Uranus, for example, which would gravitationally be very similar to a planet, the thing is that they would be very difficult to detect, although it would have an advantage, if planet number 9 exists both Like a primordial black hole, it surely has a lot of small ones orbiting, because the further we get from the Sun, the easier it is for a planet to capture moons. Pluto has five, for example, in a huge one, which is Charon, but it also has four others. further, And that is a dwarf planet.

This theory of planet number nine was proposed by researchers, Mike Brown and Konstanin Batygin, who work at the California Institute of Technology, the name, the thing about planet number nine, it doesn't have a name, it doesn't have any mystery, because it would be planet number nine of the Solar System, before we were nine with Pluto, but then, as they have changed it, we went to eight, so now, if this one is found, it would be nine, if another one is found, more distant, then it would be ten, it has no no mystery. Occasionally they also call it, in some media, planet X, planet X is the symbol of the unknown or search and at the moment, it is a mystery, if that planet exists it is speculated that it would be an ice giant, expelled from the region of the known planets during the chaos in the early days of the formation of the Solar System. By the way, planet M, don't say it too much, because astronomers don't like that name, for example, Neptune was discovered thanks in part to an alteration in the orbit, An alteration was detected in the orbit of Uranus. In summary, we have a fact and that is that there is an alteration in the orbits of these objects, the question is what caused it, explanation number one, that there is another planet that caused that orbit, why is that explanation number one? , because it is the same one that worked with the Neptune theme. Neptune caused or causes a small deviation in the orbit of Uranus and that was the reason why Neptune was searched for and has ended up being found, there may be other theories as initially mentioned.