Why is Pluto not in the solar system?

In Roman mythology, Pluto (the Greeks called the leader of the underworld Hades) is the leader of the underworld. The planet got this name (instead of other suggestions) because it is so far away from the sun that it has been silent in endless darkness. Coincidentally, the two letters at the beginning of Pluto (pluto) are the abbreviations of the discoverer Percival Lowell. . Pluto was discovered in 1930 by a lucky coincidence. A calculation "assertion" that was later found to be erroneous: based on studies of the motions of Uranus and Neptune, there was a planet after Neptune.

Clyde W. Tombaugh of the Lowell Observatory in Arizona, USA, did not know about this calculation error and made a very careful observation of the solar system. However, it was precisely because of this that Pluto was discovered. After its discovery, people quickly discovered that Pluto was too small and had different orbits from other planets. Research on the unknown planet (Planet X) continues, but nothing has been discovered. If the mass of Neptune calculated by the Voyager 2 spacecraft was used, then the other mass difference disappeared, and there would be no tenth planet.

Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can only observe its superficial appearance. Fortunately, Pluto has a satellite, Charon. It was only through good luck that it was discovered. This was discovered in 1978 when it was just on the edge of its orbit as it moved into the solar system. So it is possible to observe many of Pluto's movements through Charon and vice versa. By carefully calculating what parts of objects are covered when, and looking at light curves, astronomers can create rough maps of light and dark areas in both hemispheres. The radius of Pluto is not yet clear, and the JPL (Jet Propulsion Laboratory) value of 1,137 kilometers is believed to have an error of ±8, or almost 1%. Although the total mass of Pluto and Charon is well known (this can be determined by precise measurements of the period and radius of Charon's orbit and Kepler's third law), the respective masses of Pluto and Charon are not. It's hard to know for sure. This is because in order to calculate the mass separately, we must measure more accurately the center of mass of the Pluto and Charon systems when they are running. However, they are too small and too far away from us. Even the Hubble Space Telescope cannot measure this. Nothing can be done. The mass ratio of the two stars may be between 0.084 and 0.157. More observations are underway, but the only way to get truly sophisticated data is to send a spacecraft there.

Pluto is the second most contrasting object in the solar system (after Iapetus). Exploring the causes of these differences is one of the first goals of the planned Pluto Express mission. Pluto's orbit is very unusual, sometimes coming closer to the sun than Neptune (from January 1979 to February 1999). The synchronous motion ratio of Pluto and Neptune is 3:2, that is, Pluto's revolution period is exactly 1.5 times that of Neptune. Its orbit is also farther away from other planets. So although Pluto's orbit appears to cross that of Neptune, it actually does not. So they never collide (explained in great detail here). Like Uranus, Pluto's equatorial plane is almost at right angles to its orbital plane. The surface temperature of Pluto is not very clear, but it is probably between 35 and 45K (-238 to -228°C).

Pluto's composition is not yet known, but its density (about 2 grams/cubic centimeter) suggests that Pluto may be a mixture of 70% rock and 30% icy water like Triton. The bright part of the surface may be covered with some solid nitrogen and small amounts of solid methane and carbon monoxide. The composition of the dark part of Pluto's surface is unknown but may be some basic organic matter or photochemical reactions triggered by cosmic rays. Little is known about Pluto's atmosphere, but it is probably composed mainly of nitrogen with small amounts of carbon monoxide and methane. The atmosphere is extremely thin, and the ground pressure is only a few micropascals. Pluto's atmosphere is probably gaseous only when Pluto is near perihelion; during the rest of Pluto's years, the atmospheric gases condense into a solid state. Near perihelion, part of the atmosphere may escape into the universe, or even be attracted to Charon. The planners of the Pluto Express mission wanted to reach Pluto while the atmosphere was slipping and solidifying.

The unusual orbits and similar sizes of Pluto and Triton suggest a historical relationship between them. Some people once thought that Pluto was a satellite of Neptune in the past, but this is now believed not to be the case. A more common theory is that Triton, like Pluto, originally moved freely in an independent orbit around the sun, and was later attracted to Neptune. Triton, Pluto, and Charon may be surviving members of a large class of similar objects, with the others being repelled into the Oort cloud (matter outside the Kuiper belt). Charon may be the product of a collision between Pluto and another celestial body, just like the Earth and the Moon. Pluto can be observed with non-expert telescopes, but it is not easy.

Mike Harvey's planetary sky map can show the nearest position of Pluto (and other planets) in the sky, but it will take more detailed sky maps and months of careful observation to actually find Pluto. Accurate astronomical maps can be drawn by planetary programs such as "Brilliant Galaxy". On August 24, 2006, after discussions at the Prague Conference, the planet was excluded from the ranks of the nine planets and officially downgraded to a dwarf planet.