This is a mysterious celestial body, comparable in mass to the Earth, which revolves around the Sun in a very distant orbit.
1. The assumption that beyond the orbit of Neptune there may be some other celestial body whose gravity perturbs the orbits of Neptune and, to a lesser extent, Uranus, appeared soon after the discovery of Neptune in 1846. Astronomers searched for the mysterious planet for a long time and very hard until Clyde Tombaugh discovered Pluto in 1930. It quickly became clear that the new planet was somehow very dim. Either its albedo (reflectivity) is similar to road asphalt, or the planet is very small. It was still impossible to reasonably prefer one of the versions at that time, astronomers considered that Pluto was large, but dark, since this assumption made it possible to explain the visible anomalies of the motion of other planets.
2. In 1978, Pluto's moon Charon was discovered. Observing the movement of two celestial bodies around each other (they revolve around a point located outside of both of them) finally made it possible to accurately measure the mass of Pluto. It turned out that it is about 500 times smaller than the Earth. It became clear that Pluto's gravity could not affect the motion of Neptune, let alone Uranus. The search for Planet X is back on the agenda.
3. The meaning of X in the name was different at different times. Initially it was "X" - a symbol of the unknown and mysterious. This was the name of the still undiscovered planet by the American astronomer Percival Lowell in 1906. He spent the next ten years looking for her until his death. What is remarkable, he found Pluto in 1915, but he simply did not notice it in the photograph. After the discovery of Pluto, "X" began to be understood as 10, because this planet was supposed to be the tenth in the solar system. Finally, in 2006, the International Astronomical Union reclassified Pluto, transferring it from ordinary planets to dwarf ones, and thus reduced the number of planets in our system by one. After that "X" became "X" again.
4. In 1992, Miles Standish, having analyzed the data obtained during the flight of Voyager 2 past Neptune in 1989, specified the mass of the giant planet. It turned out to be half a percent less than previously thought. The introduction of this amendment into the equations of the 19th century fully explained all the anomalies of the orbital motion of Uranus. On this side, the need for Planet X has disappeared.
5. Assumptions about the existence of a mysterious large-mass Trans-Neptunian object are now based on the analysis of the orbits of bodies in the Kuiper belt and even more distant ones. The parameters of the orbits of Sedna and some other distant bodies, from the point of view of modern astronomy, are inexplicable. The version suggests itself that they were pulled so far from the Sun by the attraction of some not yet discovered object. It may be another star that once passed in the immediate vicinity of the Sun, or it may be a still undiscovered planet, orbiting very far from our star. More recently, 2015 TG387 was discovered - a small object of 300 kilometers in diameter, orbiting the Sun in an unprecedentedly elongated orbit - in aphelion it is located at 2300 AU. from the luminary.
6. Another fact that needs to be explained is the sharp drop in the Kuiper belt at a distance of 48 AU. from the Sun, after which there is nothing in space. The suspicion arises that the space there has been cleared by some massive body.
7. If the planet X exists, then the temperature on its surface of the glint is to absolute zero, which excludes the existence of liquid water, atmosphere and life. Therefore, if life exists there, then it is radically different from the concept of "life" to which we are accustomed.
Promotional video: