Some say it doesn't exist and some think it's even a black hole. The Solar System may have one more planet: what the experts say.
The Solar System planets are eight, but maybe not. Because scientists have speculated the existence of a ninth planet, which then could join the club along with Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Pluto is not part of the group, as it is explained in this article, with which we try to answer a question not at all trivial: how many planets are there in the Solar System?
Where is the alleged ninth planet of the Solar System
An additional complicating element is given by an alleged new planet, not observed but deduced by scientists based on the behavior of other celestial stars: in particular, it is the unusual behavior of a small grouping of icy bodies. The behavior of the objects of the Kuiper belt seems to be affected by the gravitational effects of "something": but what, exactly? Well, for scientists it could be a sensational ninth planet, located in the outer belt of the Solar System and escaped the analysis of astronomers, who every day point their eyes to the sky in search of new insights.
It goes without saying that this would be a discovery far from irrelevant.
What's the evidence to support (and against) the existence of a new world "near" Earth
At the moment, the strongest argument against the existence of the "extra planet" is actually much simpler than the lucubrations about its existence. It sounds something like this: if this new celestial body exists, why can't we find it? On the other hand, based on calculations made by those who hypothesize its existence, the new world would have a mass five times that of the Earth, and would be located at a distance about 10 times greater than that between the Sun and Neptune.
Other scientists argue that yes, Planet 9 exists, but we can't see it because it is a primordial black hole. Having a black hole as a "neighbor" would perhaps be just as sensational as it would be to discover a planet never seen before.
Deductive reasoning, able to indirectly demonstrate the discovery of a planet, should not be underestimated: this is exactly how we were able to discover Neptune. John Couch Adams and Urbain Le Verrier noticed in fact that Uranus seemed to be pulled by an invisible planet, that shortly thereafter would be discovered.