Pluto is so different from the giant planets that since Clyde Tomb discovered it in 1930, various hypotheses about the origins of Pluto have been put forward many times.
It is known that Pluto's position with the solar system contradicts the empirical rule of Titius-Baudet, which predicts for it a large axis of the orbit of 77 a.u. (at the actual value of 39.4 a.u.). For Neptune, too, there is no good match: 30,1 a.u. instead of the predicted 38,8 a.u. But the position of planetary orbits is actually determined by the theory of resonances, and the rule of Titius-Bode is his special case. The orbital positions of Neptune and Pluto correspond to 1:2 and 1:3 with respect to Uranus and, as a result, 2:3 for Pluto's orbit with respect to Neptune.
In 1936, when we did not yet know that Pluto was a double planet, it was hypothesized that it was once one of Neptune's satellites, but as a result of its approach to an unknown planet, it was thrown out of the system, and another Neptune satellite, Triton, went into an unusual reverse orbit. It was even supposed that from the current orbits of Pluto and Triton it was possible to calculate the mousse and orbit of an unknown planet.
Pluto really looks like a satellite of Neptune Triton. Although critics have argued that the orbits of Neptune and Pluto did not intersect at all, the idea of a close relationship between these bodies gradually spread. In 1984, there were even calculations of the joint origin of Triton and Pluto, which considered the possibility of Neptune capturing a massive protoplanetary body (planetesimals), which had disintegrated into Triton and Pluto. The calculations gave a completely different result. They showed that if such a catastrophe had occurred, the second, "plutonium" half would have probably been released outside the solar system and that the Triton and Pluto are likely to be of independent origin.
Nevertheless, the external (and possibly internal) similarities between the two bodies are undoubtedly similar when one considers the similarities in composition, mean density, size, atmospheric characteristics and distances from the Sun. Another issue is the likelihood of a catastrophe of this magnitude and the possibility of restoring its details to the known orbits. Although such catastrophes are indeed possible, the origin of Pluto is no longer associated with Neptune, but with the belt of transneptunian objects (the Cooper belt), which will be described below. Formal recognition of the "inferiority" of Pluto as a planet was the decision of the IAS (2006) to introduce a new family of bodies of the solar system - dwarf planets, the prototype of which was Pluto.
Pluto and Haron
Because of the great eccentricity of its orbit, Pluto was closer to the Sun than Neptune from 1979 to 1999. Since its discovery in 1930, Pluto has still been north of the ecliptic and is now about 4 a.u. above it.
As early as the 1960s it was established that Pluto's luster was changing with a period of 6 days 9 hours and 17 minutes. This value was taken as the period of Pluto's rotation and was not mistaken. But it turned out that the same importance is also attached to two other southern periods associated with Pluto. In 1978, it was proved that the slightly elongated shape of Pluto's image in the photographs meant he had a satellite. Both bodies have similar sizes, so it is more correct to call them a double planet or a system of two planets. Nevertheless, it is customary to talk about Pluto and his satellite Haron.
With the Hubble Space Telescope, for which there is no Earth's atmosphere problem, it was the first time that we managed to take a picture of Pluto and Haron separately. They are so close to each other (<1˝) that at first they could be separated from the Earth only by the method of speckle-interferometry, but in 2000 new ground telescopes with active optics allowed to divide confidently Pluto and Haron. The period of 6,387 days was also the period of their mutual circulation around the common barycenter. The total mass of the Pluto Haron system (0.0205 Earth-mass) was measured using the measured period and orbit radius. The ratio of their masses (1/7) was higher than that of any other satellite-planet pair.
The orbital plane of the components is turned to the Earth so that in 1985 they began to systematically eclipse each other (cover) for several hours. The full phase of the eclipses was in 1988 and ended in 1991. The next period of eclipses will be repeated only after 124 years. The decrease in the brightness of the eclipses was 4 and 8% alternately, from which it was concluded that the surface of Haron is 30% darker than that of Pluto. Now the Hubble telescope has already produced fuzzy images of Pluto.
From Pluto, the Sun's disc is indistinguishable to the naked eye. Therefore, the Sun shines like a dazzling star there, dimly illuminating the surface. However, this illumination should be enough for television shooting and even reading.