Mercury is 4878 kilometers in diameter, while Pluto is only 2370 kilometers in diameter, which is more than twice as large. The mass of Mercury is more than 20 times that of Pluto.
But mercury is really small, smaller than two satellites in the solar system. These two famous satellites are Ganymede, with a diameter of 5262 kilometers; Titan has a diameter of 5 150 km. Callisto is similar to Mercury, with a diameter of 4,840 kilometers. Our moon is 3476 kilometers in diameter, slightly smaller than Mercury, ranking fifth among the satellites in the solar system.
Pluto is even more pitiful. There are seven satellites bigger than it. In addition to the above, there is Io, with a diameter of 3,660 kilometers. Europa, 3132km in diameter; Triton, with a diameter of 2520 kilometers.
The main conditions of a planet should be: 1, and it must be a celestial body orbiting a star; 1, must be large enough, its own gravity is enough to make its surface spherical, with a diameter of at least 800 kilometers and a mass of more than 5 billion tons; It can empty other celestial bodies in its orbit.
It is clear that mercury fully meets the above conditions. Mercury is a celestial body revolving around the sun. This is a nearly perfect round planet. Its orbit has been emptied, not even a satellite. Its mass is 3.3 *10.23 kg, far exceeding the mass of 5 billion tons. Therefore, Mercury is a veritable planets of the solar system.
Pluto was discovered in 1930. Because of the poor early observation methods, people began to think that it should be a bigger planet, even bigger than the earth, so it was listed as one of the nine planets in the solar system.
Later, dwarf planets were discovered in the Kuiper Belt, namely ceres, ceres, birds and pregnant stars. Some of these dwarf planets are comparable to Pluto. For example, Mars is 27% more massive than Pluto.
In this way, at the International Astronomical Union in 2006, Pluto was voted out of the Planetary Team, ending its 76-year career in the Planetary Team and being listed as a dwarf planet. In addition, Ka Rong is not considered as a satellite of Pluto and should be classified as a dwarf planet. In this way, there are seven or eight famous dwarf planets.
The obvious differences between Pluto and other planets are: 1, which is too small, smaller than many satellites; 2. The track eccentricity is too large, reaching 0.256, and the perihelion is 7.3 1 100 million kilometers, which is 1.66 times different from the perihelion's 4.43 billion kilometers. 3. The orbit will sometimes enter Neptune orbit for internal measurement. Some people suggest that it was originally a Neptune satellite and was later thrown out. Strictly speaking, Ka Rong is not a satellite. Pluto and Ka Rong revolve around the outer space particles of their respective stars and tidal locking each other. This can be seen as Pluto not clearing its orbit, but sharing it with other stars.
These characteristics are the main reasons why Pluto was finally excluded from the planetary team. At present, there are still some controversies in the scientific community about Pluto being kicked out of the planetary team, and there are still some voices about Pluto being re-listed as a planetary fairy.
Fortunately, after the New Horizon probe arrived in Pluto on July 14, 2065438, it obtained some accurate data and determined that the diameter of Pluto was 2,370 kilometers, slightly larger than that of Venus. In this way, although Pluto can't be a phoenix tail in the planet, it is lucky to be the head of a dwarf planet. The obvious reputation is that dwarf planets and many asteroids living outside Neptune's orbit are classified as "Pluto" or "ghost-like objects", so it should be a little comforting.
This is why Mercury can be a big planet and Pluto can only be a dwarf planet.