Why Pluto was relegated to the Nine Planets

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It is said that for planets, there are 3 conditions, and Pluto does not meet the 3rd condition, so it is eliminated. But Pluto has been relegated to a dwarf planet.

Resolution 5a IAU decides that the planets and other celestial bodies in our solar system are divided into three distinct categories in the following manner:

1) A planet 1 is a celestial body that satisfies (a) orbiting the Sun, (b) having a mass large enough to overcome solid stresses to achieve a hydrostatic equilibrium (near-spherical) shape, and (c) having a clear neighborhood relationship in the orbital range.

2) A dwarf planet is a celestial body that satisfies (a) the shape of (a) orbiting the Sun, (b) having a mass large enough to overcome solid stresses to achieve hydrostatic equilibrium2, (c) having an unclear neighborhood of the orbital range, and (d) not being a satellite.

3) Other celestial bodies orbiting the Sun3 are collectively referred to as "small bodies of the solar system".

Resolution 5b In section I of resolution 5a and in footnote 1, insert the word "classical" before the word "planets". Namely:

1) A classical planet 4 is a celestial ......

IAU Resolution: Pluto.

Resolution 6a IAU further decides:

Pluto is a dwarf planet by the definition above, and serves as the prototype for a new class of overseas celestial bodies.

The following statement in Resolution 6b was added to Resolution 6a:

Such objects are called "plutonian objects".

Because Pluto is different from the other eight planets in many ways: Pluto is too massive; The orbits of the eight planets are all approximately circular, but Pluto's orbit is elliptical and has an intersection with Neptune's orbital plane; The orbital planes of the eight planets are almost all in the same plane, but the orbital planes of Pluto have a great inclination ......... with the orbital planes of the eight planets

Because it was a mistake to be listed among the nine planets before. Now it's just a matter of correcting the mistakes1

Pluto can only be considered a dwarf planet. It is too small for the other eight planets

Because Pluto is too small to be considered a big planet!

This is due to the comprehensive consideration of orbit, volume, composition, revolution period, etc.

Although it has a certain mass, and although it orbits the sun, it cannot use gravity to clear the surrounding stars - it does not meet the definition of a new planet, so ...

Additional upstairs: The orbit is different, and there are asteroids in the solar system that are bigger than it!

Soon after Pluto named the nine planets, there was controversy, because there is an asteroid belt called the Kuibo Belt in the outer reaches of the solar system, and Pluto is so close to it that it can be said that it is a member of the Kuibo Belt, and asteroids larger than Pluto are constantly found in the Kuibo Belt.

In addition, the distribution of planets in the solar system is that terrestrial planets are close to the sun, Jupiter-like planets are on the periphery, and Pluto, a terrestrial planet, suddenly appears on the outermost side of all planets.

In the universe, is it nine planets or eight planets, and why was Pluto removed?

Pluto was removed from the list because the mass of Pluto was so small, not even comparable to the mass of some moons, that Pluto was removed from the list of planets.

Pluto is not big enough to gravitationally clear other objects near its orbit, so it becomes a dwarf planet.

Pluto's inclusion in the list of major planets was caused by a mistake that was corrected in 2006. According to the definition of the International Astronomical Union, for a celestial body to be called a planet, it needs to meet three conditions: it orbits the Sun, it is so massive that its gravity is large enough to make it a sphere, and it is capable of clearing its orbit of other objects except satellites.

Pluto is able to meet the first two criteria, but not the third. As a result, Pluto was removed from the nine planets and demoted to a dwarf planet.

Because Pluto is so far away from the Sun, the Sun's gravitational pull can no longer completely restrain its movements.

The International Astronomical Union has stripped Pluto of its status as the main planet in the solar system, mainly because there is little difference between Pluto and the numerous objects waiting to be discovered outside its orbit, and these objects are too different from the major planets that have been identified. This led to Pluto's expulsion from the solar system. The International Astronomical Union was officially established in Paris, France.

To date, it has 79 member countries and is the largest international astronomical organization in the world. It governs the naming and classification of all celestial bodies.

<> traditionally, the Astronomical Union holds a plenary meeting every three years. In August 2006, the 26th General Assembly of the International Astronomical Union was held in Prague, the capital of the Czech Republic. It stands to reason that this highly specialized astronomical meeting will not attract any attention from outsiders.

But on August 24, the conference suddenly became the focus of global attention. Because that day, after a vote of more than 2,000 astronomers, the International Astronomical Union passed a resolution to remove Pluto from the list of planets.

You may ask, "Who is Pluto bothering?" Why kick it out of the star system? "I'll tell you the whole story.

The first problem with Pluto is that it has too little mass. For a long time, Pluto's mass could not be accurately measured due to its distance. In the early 30s of the 20th century, some astronomers estimated the mass of Pluto by analyzing the influence of Pluto's gravitational pull on the orbits of Uranus and Neptune, and they came up with a mass that was about the same as the mass of the Earth.

By the end of the 40s of the 20th century, Kuiper had recalculated Pluto's mass based on observations from more precise telescopes and found that his previous results were wildly wrong. Kuiper's research shows that Pluto is only one-tenth the mass of Earth and about the same as Mars.

But this method based on mechanical analysis proved flawed, as it assumed that Pluto was responsible for all changes in the orbits of Uranus and Neptune. In fact, there are many other objects beyond Pluto's orbit. It wasn't until the 70s of the 20th century that astronomers finally found a more reliable way to calculate Pluto's albedo, or its ability to emit sunlight.

This causes Pluto's mass to plummet again, less than 1% of Earth's mass. The latest research shows that Pluto has only about the mass of Earth. So here's the problem.

The Moon may be 81 times more massive than the Earth, but it is still a satellite. Pluto's mass is only Earth's. Why should it be a planet?