Why Pluto was removed from the Nine Planets

In the new definition, in orbit around the Sun; There is a large enough mass to overcome the solid stresses to achieve a hydrostatically equilibrium shape (nearly spherical); Celestial bodies that can empty the area near their orbit are called "planets", and as far as current observations are concerned, only eight planets meet these definitions.

Even if the first two conditions are required, a celestial body that has not been cleared of the area near its orbit is not called a planet, but is given a new name as a "dwarf planet", which is cleared from the range of "planets"; The definition of dwarf planets specifically states that a dwarf planet is not a moon, so Charon, the moon of Pluto, which was previously speculated by experts, is not included in the list of dwarf planets.

To put it simply, Pluto is so different from the other 8 planets that it has a lot in common with many other types of objects (as qdmaomao says, the diameter is too small, and its orbits intersect with other planets), so it is classified as a different kind.

He is too small to belong to the planets, only to the dwarf planets.

The diameter is too small, and the orbit intersects with other planets.

Because it's so small

This is not very standardized, check it yourself for scientific points.

The nine planets, Pluto, were removed because they did not fit into the planetary world.

Scientists believe that Pluto does not fit into the planetary theory. With regard to planets, it is necessary to meet three conditions: orbiting the Sun, hydrostatic equilibrium, emptying other celestial bodies on the trajectory.

However, Pluto's mass is not large enough to form a hydrostatic equilibrium, and its trajectory is interspersed with Neptune's trajectory that does not meet the third condition.

As a result, the scientists decided to downgrade it. The downgraded Pluto is on a different trajectory from other celestial bodies, and Huaicha walks quietly in the dark. However, in the future, it is no longer lonely, and many studies will make it the focus of astronomy, such as the discovery of the "snow-capped mountains" on Pluto in 2015 and the discovery of the existence of lead-down skin energy in low-temperature volcanoes in 2016, which will make this planet that has been silent in the dark for a long time shine.

The internal structure of Pluto

Because the decay of Pluto's radioactive elements will eventually heat the icy material, separating the rocks from the ice, scientists believe that Pluto's internal structure is unlike any other, with rocky material settling into a dense core surrounded by a water ice mantle. New Horizons previously estimated the diameter of its core at 1,700 kilometers, which is 70 of Pluto's diameter.

It is possible that this heating will continue, forming an underground ocean of liquid lead water 100 to 180 km thick at the boundary of the mantle. In 2016, scientists simulated the impact thought to have formed the Sputnik Plain and suggested that it could be the result of liquid water rising from below after the collision.

This means that there is an underground ocean at least 100 kilometers deep. Pluto does not have a magnetic field. In June 2020, astronomers reported evidence that Pluto might have had an internal ocean when it first formed.

The above content refers to Encyclopedia - Pluto.

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The General Assembly of the International Astronomical Union passed a resolution on the 24th to "expell" Pluto, whose status is controversial, from the ranks of planets in the solar system, and the number of planets in the solar system has been reduced to 8.

From now on, Pluto, a celestial body that wanders the edge of the solar system, will only be called a "dwarf planet" along with some other "siblings" of about the same size.

The day's rally began with the beautiful singing of the four singers, which ended with a fierce argument and ended with a "milestone" resolution.

The convention was always tense and tense. Until the vote, some astronomers seized the last chance to express skepticism. Standing in front of a microphone erected in the aisle of the auditorium, they asked Ron Eckes, the president of the International Astronomical Union, who was sitting on the podium, to revise the draft resolution again.

One astronomer even asked to revise one of the punctuation marks.

At the time of voting, the opposition between the two factions was evident. Astronomers waved their ballots in a seditious way to encourage more people to join them, including Aix, a strong supporter of Pluto.

According to the new definition adopted that day, "planet" refers to a celestial body orbiting the Sun that has a gravitational pull sufficient to overcome its rigid force to make the body spherical and capable of clearing other objects in the vicinity of its orbit. Pluto, on the other hand, does not meet this definition because its orbit intersects Neptune. The resolution adopted by the General Assembly says:

Solar system) planets include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. ”

Pluto is a "dwarf planet," the resolution said. A "dwarf planet" is a celestial body that also has sufficient mass and is spherical, but cannot clear other objects in the vicinity of its orbit. The resolution also identifies a class of outer Neptune objects and makes Pluto a "typical" representation of that group.

Pluto shouldn't be a planet, every astronomer should know it," Ian Howworth, an astronomer at University College London, told Xinhua after the resolution was passed. Like most astronomers present, he voted against Pluto.

But for Aix, president of the International Astronomical Union, the result of the vote was "a pity". In his opinion, "dwarf planets" should also be included in the list of planets.

Fang Cheng, vice president of the International Astronomical Union and academician of the Chinese Academy of Sciences, said in an interview with Xinhua News Agency: "Pluto is indeed the focus of this planetary definition process, and many scientists believe that it should not be a planet. ”

Although there is still no consensus among scientists on this issue, the emergence of a new definition of planets is a "historic" moment. Aix said that the study and discussion of planets will continue in the future, but the creation of this definition is a milestone in astronomical research.